yes generally only saltwater uses sumps, thought I don't see why FW couldn't, I love keeping the heater and such hidden.
So there are basically two ways to do the overflow box, add one to a regular tank like this one (no drilling required)
Or to get your tank drilled, you can order them like this. Basically the water flows through the cracks there at the top and there is a tube at the bottom of the tank that is drilled through and plumbed in.
The drilled can also look like this (this is our big tank, It will be built into a wall so the overflow box wont be seen, our sump is also beside the tank instead of underneath it)
This one is still drilled through the tank but all pipes are on the outside.
This photo shows it a bit better. (second pipe still needs to be plumbed in)
They are gravity fed, I suppose you could try with two pumps but the problem would be trying to get them to pump at the exact same rate, near impossible I'd say. One would always be slightly faster and end up emptying the one area before the other could fill.