How to keep two water tanks have maintain the same water level?
Note: this answer is for pressurized plumbing of two hot water tanks. Rereading the question I'm pretty sure it's an incorrect assumption so I'm only leaving it here in case it helps someone with this problem.



The typical way to do this is to run the two tanks in serial instead of parallel. You would connect the output of one tank to the input of the other. I've seen this with a solar and electric tank combination. The solar tank was there to preheat the water, and the electric was the second tank that would run on cloudy days. Connecting the two tanks is quite complicated if you need the ability to bypass either tank. Here's a quick ascii art of what I've seen:

         +----X------+--- Hot
         |           |
Cold --+------X----+ |
       | |         | O
       O +----O----+ |
       | |         | |
       C H         C H
       WH1         WH2


The X are closed valves, O are open valves, C is cold input to a WH, H is hot output from a WH. The valve setup that is shown here is for the cold input for the home to go to WH1, back out the hot, into WH2, and back out the hot to the hot line for the house.

You could simplify this if you don't need the ability to isolate one of the water heaters:

Cold --+             +--- Hot
       |             |
       O +---------+ |
       | |         | |
       C H         C H
       WH1         WH2


The nice thing about having the water heaters setup in serial is that you can completely turn one off and still have hot water available to the entire home, just in lesser quantity. And in times of high demand, having both heaters running can get the water back to the high temperature twice as fast.