It's a drywell... water goes in and is absorbed into the soil. If you are in an area with sandy soil, this is usually ok. In heavy rain seasons, the water table rises as more and more water is absorbed into the ground. As things return to normal, the water table drops.
If you're interested, Google for "water budget" or "groundwater budget" and you should find some more detailed explanations.
I'd recommend monitoring the situation. If it seems damp often, start to do what you can to direct stormwater away from the house. Put extensions on gutters and regrade around the perimeter of the house.
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.
Best Answer
Put the second cistern above the first cistern, and let gravity move the water from the second into the first, leaving the pump on the first. Use a valve on the pipe from the second to the first to prevent overflowing the first when both are full/being filled.