I think this is never an exact science. First check your valves on both feed and return. Make sure none have been accidentally closed at all. If all 4 are wide open, start to experiment. If there is one tank staying way more full than the other, try incrementally closing the return valve ( very small adjutments) to that tank. This should return more fuel to the lower tank. Trial and error. When it is critical, as in long trips, run one tank at a time.
If you have filters on each tank, it could be a restriction as well
If it is gas powered, you only have feed lines to play with. Choke off the tank that is burning more fuel till you even them out.