Well, I'm no expert on this sort of thing, but there are a couple different ways to do something like this. Drill 1 hole for your overflow and 1 hole for the return. You don't need a ton of flow for your return. 5-10 times turnover on your sump is good enough. I'd imagine a 3/4" for the overflow with a 1/2" return with locline to split the return. I'd probably go with a mag 3, but a mag 2 might be enough depending on how far away the sump is.
Another way would be to cut an oversized overflow and run the return back up through the same hole. I'm not very familiar with what size to cut the overflow, but I'd imagine maybe a 1.5" overflow with the same 1/2" return running back up inside the overflow piping. Hopefully that makes sense.