I have 2 waters on a boat, 1 is 50L and the other is 35L. I have no choice because of space !
I have tried plumbing them in series using the smaller tank as a feeder into the larger tank but I got back pressure at the taps, quite a lot actually and even with the water system turned off, hot water would come from the cold taps for quite a long time.
Can you help me on this subject.

Warren

I got back pressure at the tapsPleaswe explain what you mean by "back pressure". Are you talking about some sort of suction?

hot water would come from the cold taps for quite a long time. Are you saying that after you close the cold water tap hot water continues to run? Can you be more descriptive? Back to you, Tom

Install a check valve that will take care of hot water back flow. Also, make sure you have installed Pressure Relieve Valve on EACH water heater tank. Back to you. Milo