You may wish to go with two units that can be linked together, therefore fire and modulate together as necessary.
Take a look at the Takagi TK3 for instance that comes with the communication cable.

Course twice the money and you need a big gas supply, beyond the standard 250cu ft residential meter and piping.

Sorry, but 5gpm per head is ridiculous. I have a hard time buying that it's even legal to produce that as a standard item, considering 2.5gpm has been the standard since 1992. But I'm sure they have or think they've found some loophole. And no one's enforcing that sort of thing for the past 8 years, anyway.

You can have a perfectly enjoyable shower with 2.5gpm per head. Heck, you can have a just-fine shower with less flow than that.

That you're considering spending $4-5k just on tankless heaters to heat water for your "splurge" shower ought to be a little bit of a reality check. Not to mention the gas bill that'll go along with using them.

I have no problem with having 2 heads- we put in a great big shower that has them. But they're Hansgrohe with the flow restrictors in place, and they only both get used when 2 folks are in there.

Here is a link that might be useful: Down the drain

Got to agree with Thull:

There is a technical answer to your question... but there's a much better common sense answer: Don't do it.

Here's a better idea, just write me a check for half the amount of money you would have wasted on a dual heater, oversize gas service, water wasted and gas wasted down the drain over the life of the shower. A check for $15k should about do it.

I tried one of those showers once with the big multi heads and body sprays. It wasnÂt too enjoyable and you can't stand it for very long. With all the water you can't see, open your eyes or do much else. It's a fad that fades for anyone that experiences it.

Reminds me of the new homeowner that was so proud of his new shower under construction, after seeing his 45 GPM shower and all the storage tanks in the basement, I asked him; when do you plan on breaking up the marble floor to replace the one 2" drain with a minimum of 2-3" if not more to handle the water?

Oh yea besides, can a 1" water line 100' to the street supply 45 GPM?

-Deer in the headlights

I have a dual shower in the master bath, two Grohe showerheads with thermo-balance controls and a Bosch Aquastar 250 NG tankless water heater for the whole house.(one of the poorest rated products on the market) They work fine if you use them together, as long as the washing machine is not filling at the same time. I have had the setup for 4 years. My location is coastal Northern California, where incoming water temperatures are usually around 50 F.

The only issue with the tankless setup is every time the unit shuts down and starts back up, about a gallon of cold water goes through the unit before the burner heats it back up to the set temperature. This was annoying as heck, so I added a 6 gallon electric heater next to the tankless, and the hot water coming out of the tankless goes into the 6 gallon electric tank, which serves basically as a mixing and tempering device. It stays set at 110 F, and is replenished with hot water all the time from the tankless unit, so the electricity consumption is very low.

I am assuming that you will have a thermostatic valve and volume controls to give you your 10 gallons a minute and because you will have volume controls you can certainly lower the volume while maintaining your temperature and operate seperately as well. However, you will still need to size your instantaneous hot water heater and depending on where you live you could possibly get by with a Rinnai 9 gpm unit, but changes are you will need to "stack" them to get what you need if someone else is showering, etc.