The best way to do it is keep them together and interlock the control/operation. Since this can't be done sounds like water flow is backfeeding, install spring loaded check valve on each unit on the cold water inlets

is the backfeeding confusing the heaters?

You actually want to run them in parallel. Right now, there's nothing to control where the hot water flow comes from at a given fixture, besides the piping losses (length/size of pipe, fittings). If you have a bigger line to/from the bigger heater, and it's closer to the cold water entrance to the house, hot water will want to come through it preferentially.

You can get some brands (I know you can for Noritz) where they communicate between 2 heaters in parallel. But I believe that's really intended for where they're sitting next to each other.

With them on the opposite sides of the house, you'll probably need to do what we did. We have a tankless on one end of the house for 2 bathrooms. The older gas tank heater is in the garage on the opposite corner of the house. In the crawlspace, there's a closed valve on the hot water line in between the 2 bathrooms and the rest of the house.

Basically, this zones the hot water system. If either heater is out, I can open the valve and feed the whole house. But the 2 heaters don't fight each other under normal circumstances.

To get your smaller heater to work, you need a closed valve somewhere. It might be on the extension/connection you mentioned in your OP. But that depends on how much stuff is on the 2 systems.

I may be wrong, but I think zl700's check-valve solution would still lead to zero flow through one unit most of the time. That'd be the smaller one, since it sounds like it's running the least now.

The heaters (good ones) have motorized inlet valves used for temp/flow control, most sit open waiting for flow to start, since water seeks lower pressure, as the closest unit sees flow, the higher pressure migrates.
In your case, the theory is the closet unit to fixture should see flow, and once capacity is reached the flow should divert from other unit.

However:

On a proper twinned install when the units are controlled in a multi-unit install and cascaded, one units water inlet valve remains closed, until near 1st unit capacity is reached, then second water valve opens.

The dead giveaway of reversed flow is when you shut one off everything it is fine. The luke warm temps you are experiencing is literally a cross-connection causing a mixed water downstream.

Install the checks, or put two matching controlled units side by side
What THull describes is isolating units from each other with a manual valve, this can be done with proper inlet checks so that your not handicapped by single unit flow, as he/she is.

thank you I will give the valves a try.

Last night I put one check valve on the inlet side of the smaller tankless 3.5 GPM but the larger 7.5 GPM still dominates the little one unless I try to fill a tub or run a shower and then the both work together fine. I was hoping that since I had two seperate systems in the past I could go back to that in a way.

When the plumber joined my two systems he ran a 3/4 inch line to the kitchen side so the water takes about two min. to get hot from the larger tankless at the kitchen sink. The smaller tankless is more than twice as fast since it has less pipe to flow.

So my question is if I put the other check valve on the 3/4 hot water line that joins the two systems with the flow not allowing the larger heater to push into the kitchen area will the smaller heater take care of that area but still come on when I want to fill the tub or run two showers like it does now. If so I would be able to set the temp to 135 on the smaller tankless for dishes in the sink etc. and set the larger at 115 for showers. In the end I would use less enegry because I am running the smaller unit for the smaller tasks or if I need more than the 7.5 GPM.

So will this work? Do I need a third check valve for the inlet side of the larger tankless too?

Thank you for your help so far and for the forum I have used it for many things during my remodel.