if you want multiple tanks in your filtration system it's best to link them together. i.e. single overflow like you said, but route everything to a common return.
For example, I did this with a 29g sump and a 10g fuge. Single Eshopps PF-1200 and split the overflows into each tank. The fuge, however, gravity drained down some PVC
pipe into the sump for the return.
There's no way to control consistently how much water is going to which tank. So, if something were to happen and 1 of your 2 return pumps failed/clogged, water would continue into the tank with the failed pump due to the other return still running and overflow your system.