Tanks aren't going to be drilled it will come over the back....
My drawing skill on the computer suck....
My drawing skill on the computer suck....
Also if the valve only closes when the power is off what happens when the pump in the filter quits or there is a leak some where but the power is still on? The water will continue to siphon hence the need for some sort of siphon brake as mentioned before or a hole drilled in all the pipe a couple inches below water lvl to allow air in to break the siphon when the water reaches that lvl.

there is alot easyier way to do this and that is a sump and overflow otherwise there are way too many problems to overcome .
drain top tank to bottom and bottom to sump , sump pump then puts water back into th top system ok yes top waste is going into bottom tank but the water will be running on a single canister so effectivly it will be anyway . a cnister doesnt clean the water perfectly every time so u will get waste coming thought the two tanks that was from the other any way .
just my oppinion but that is the ONLY way this can be done without causing serious problems and serious cost. i have a similar plan but running on four tanks and have tried in on a small setup.
I have used sumps in the past and they do have their uses but over time I have moved to canisters for their ease and how nice and clean they stay along with the inside of the cabinets.