So long as you achieve an actual 90 ft/min across the entire hood space with both blowers on, you should get good containment. Of course, as usual, the hood components have to overlap the rising and expanding cooking plumes, and the resulting capture area times 90 ft/min determines the required actual CFM. Blowers rated together to be 1.5X this value should work against typical pressure losses.

You can use two ducts, possibly 8-inch, or merge into a 10-inch or 12-inch. With enough length and space, dual silencers can be used if the blowers are on the roof. This setup may be a bit more awkward than a single duct scheme, attic obstruction wise.

In any case, MUA appropriate for the CFM and the back-draft pressure of any combustion appliances, or lacking any, minimally degrading the hood blowers' performance will be needed.

Duct velocities in the 1000 to 2000 ft/min range are preferred over higher, or significantly lower. This determines duct diameter for a given actual flow rate.

Inserts must overlap exposed wood on the bottom of the cabinetry used to cover the inserts.

I'm not sure that the finished result with labor costs will be less expensive than a single hood.

An alternative requiring less exhaust and makeup air capacity would be 2 30" induction cooktops and ovens including a combi-steam oven located in a separate stack. Miele or Bosch are examples.

Ranges are already ordered. 2 frigidaire professional. So gas is the only option at this point. I also cater so I'm used to commercial gas ovens, just never had them at home.

Cara what did you use? Two 30" inserts or one 60" insert?

@acdclosgatos we bought 2 36" inserts which will be combined into one custom wood hood. Both venting separately. House isn't finished yet. Covid really slowed everything down!

We are in the same boat getting the house done. IF you have the time then can you please answer my questions.

What brand inserts did you use?

Did you have to overlap them or modify install like previous commenter -Kaseki- stated?

Did you contact the manufacturer about proper install so the warranty wasn't voided?

Were you advised to have your exhaust hoods higher since you used a custom wood hood?

Did you use two 34" inserts or were they actually 36" wide?

Also I saw you do catering- What made you purchase the two Frigidaire 30" professional ranges?

Thanks & Happy Holiday.

@acdclosgatos we went with the victory star. We were going to get the Q7 but it's backordered for who knows how long.

I don't believe we will need to modify them, should be able to install as is. The manufacturer actually suggested going with 2 as opposed to the one.

We are placing them at 36" above counter. We didn't want to be bumping heads.

The ranges match the fridge/freezer I wanted.

Two seperate inserts will be much less effective than one single properly sized hood. This especially with consumer hoods that have too little containment area to begin with. If the kitchen is just for show to impress friends then not a big deal but if the kitchen is actually used then this is one case where it is much better to install a commercial hood.

More: https://bamasotan.us/range-exhaust-hood-faq/

And: https://www.houzz.com/discussions/5745986/commercial-range-hood-install

@opaone two hoods in this case are larger coverage and more CFM than 1 single hood and gives us better flexibility.

A separate-hoods configuration does solve the issue of wasted air flow over a larger space than is emitting effluent, so long as capture and containment are adequately achieved for each cooktop individually. If two ranges are joined up together, then each single hood won't have the needed overlap at the join, but its hood will work alone for the outer burners. Working near the join will require both hoods be utilized. This could be overcome by using a fill strip and narrow counter to separate the ranges to be centered under their respective hood apertures.

"@opaone two hoods in this case are larger coverage and more CFM than 1 single hood and gives us better flexibility."

For this to work properly you'll often have to turn on both hoods which will be quite noisy, waste energy and still not work as well as a single hood. I'd also not be surprised if this also costs more than a single commercial hood.

If you turn on only one hood, which for some single-burner operations you could perhaps justifiably do, then any effluent flowing towards the hood that's off will not get exhausted and will spread out in to the house.

You'd be far ahead to do this properly with a single hood. The result will be much quieter, more energy efficient and will work much better than a dual hood config.

Another issue that I thought about is the problem of harmonics. When there are two motors/fans in close proximity to each other you can get harmonics due to slight differences in rotational speed and balance. These can be an extremely grating kind of noise. One thing that VAH did correctly is that when they have two blowers together they spin one in the opposite direction which counteracts this without having to balance and speed match.

There is also an issue of MUA control. Commercial controllers have a built-in capability to manage multiple exhausts and MUA's to function properly, consumer do not. It can be done with a home automation system such as Control4 if you or your installer understands the exhaust system and programming well enough. A system from ElectroMN might also work if both hots are run through a single donut though it only has 3 speeds so you'll have to do some work to figure out the best solution for a multitude of combinations.

The more I think about it the worse the idea of having two hoods sounds.

In that general area there is also the issue of blower "hunting." Blowers used for residential venting have fan curves that are monotonic. That is, the flow always reduces with pressure loss. There are no flow zones where a given motor power can produce two possible outcomes.

With two single duct blowers feeding into one duct, or drawing air from a restrictive make-up air situation, I am not confident that operational conditions of different or similar powers supplied to the blowers will not cause some flow oscillation between them.

So if there are two hoods, either use two systems with very good MUA making it difficult for one hood system to change the house pressure, or use a single blower above where the two hood ducts merge to one. This is the configuration I use for my over-oven registers.

That's a good point about feeding in to a single duct. I'd assumed they'd have two totally separate exhaust ducts to the outside. Combining in to one adds another issue.

opaone & kaseki- We have two exhaust inserts over two 30" ranges. We have two 22.5" deep Vent-A-Hood inserts with separate ducting/ exhaust that are surrounded by a metal liner. We also have 2 windows in the corner with a 36" corner cabinet under them next to an 18" cabinet and then our 30" ranges.

I always open a window, turn on exhaust and then ignite the gas cook-top or gas oven.

My HVAC guy stated that if at least one of the 2 windows is cracked open a couple of inches then we won't stress the house & pull air back inside through the bathroom exhausts, dryer vents, etc.

How is there a problem with two ranges side by side with two exhaust over them?

At the moment, I would call them concerns pending further information.

First, do you have any combustion appliances connected, air wise, to the kitchen? Combustion appliances include furnaces, hot water heaters, and dryers. If they all have their own make-up air sources, then they don't count. Fireplaces count unless roaring hot and quite large, in which case they need their own make-up air even without any kitchen ventilation effects.

One of two windows cracked open a couple of inches will perhaps achieve adequate make-up air for a quite low CFM requirement in lieu of wall cracks and other dusty paths. Bathroom exhausts and dryer vents should have dampers that will not allow enough MUA for any purpose. Gas fireplaces are required to not have dampers, so only the doors limit back-drafting. That is also an undesirable path.

300 CFM, for example, is 5 cubit feet of air per second. This would require more than a couple of inches of window, even if unscreened, to avoid an undesirable pressure drop in the house when combustion appliances are present. It will certainly degrade the flow rates of the kitchen ventilation blowers, more so with VaH squirrel cage blowers used in Magic Lung systems.

What are the claimed CFM of the VaH inserts you are using? (Zero static pressure values not Magic Lung claimed equivalent values)

What is the area (or dimensions) of the air intake aperture at the bottom of the hood assembly containing the inserts?

Is there some overhang of the overall hood relative to the entry apertures of the inserts?