I would say that is a pretty good idea to have one sump for two tanks. You should probably get a bigger sump than normal so that if one of the pumps are slower than the other the sump doesn't overflow. Even thought you are making a drip system you should still change water every so often. Make sure though that both tanks used are going to have the same water chemistry like you don't want a soft acidic water tank and a hard alkaline water tank using the same sump because the water would change. Good Idea though.