Why does my shower's water temperature fluctuate several times per minute?
The small tank is a thermal expansion tank, there should be two of them. You must have a check valve if you have expansion tanks. If you have two matching water heaters, they should have been plumbed in parallel and not in series. Also, the piping associated with each heater needs to be the same length or it will cause one heater to do more work than the other. This could possibly cause a fluctuation in the water temperature. Also, make sure that both heaters are set to the same output temperature. Everything must be "balanced" in a parallel setup. Here is a diagram of what two water heaters in parallel should look like:



Here is a diagram of two heaters connected in series:



Images are from a Rheem/Ruud technical service department pdf I have on file.