You will need to have either an overflow box (or system similar to it) or drill your tank. Or you could just buy one that's pre-drilled.
This two pump setup will not work. Try it yourself on an empty tank or with pails if you need to test it. (don't worry... I did this myself, and found out the hard way). If you want a sump, my friend, you will have to accommodate the necessary equipment for it. I know you asked several times for a small overflow box that wouldn't stick out for more than a couple inches. Mine only sticks out 3" and it's an Amiracle. If you cannot drill your tank and cannot find a way to make room for an overflow box, then I would suggest either not doing a sump, or risk flooding your floor.
Sorry to be the bearer of bad news. HTH