If they are differing heights then you could just treat the lower tank as the sump. Run an overflow from a weir in tank 1 into tank 2 and run a return pumping water from tank 2 back to tank 1.
If the height difference is not very significant you will need to prime the overflow line when you first start the system running and every time you turn the return off as air will fill the overflow line.
Just like when you have a sump you need to be careful that you leave enough height between the waterline and the top of the tank in tank 2 to accomodate all the water that normally sits in the overflow, otherwise when you stop the return the overflow will empty and tank 2 will fill and cause a spill.
If both tanks are at the same or very similar heights then it’s more difficult. Trying to pump from each tank into the other will quickly result in overfilling one tank and flooding. Maybe you could use high level redundancy (e.g. sensors and float valves to shut off if the sensor fails) but it will be complicated, fiddly and ugly.
Here’s a quick thought bubble. If both tanks are at exactly the same height you might be able run a closed loop between both tanks fairly safely as long as the flow is not too high. Put two bulkheads in each tank. Connect the top bulkhead in tank 1 to a pump and then to the low bulkhead in tank 2. Connect the top bulkhead in tank 2 to the bottom bulkhead in tank 1. As you pump water from tank 1 to tank 2 that will increase the head pressure in tank 2 and water will then flow through the return connection back into tank 1. Tank 2 will have a slightly higher water level than tank 1, how much depends on the flow.
Hopefully others who have actually tried this will chime in. Good luck.
