I really appreciate your effort to help me. I feel I have not explained my situation very clearly. Sorry for the confusion. I have four holes in the tank, two on each side. They are ALL the same size. The hole size does not matter, the bulkheads for both the smaller return plumbing and the larger overflow standpipe will fit in the same size hole. I have mud and musk turtles in the tank so I don't wish to create a torrent of current. Perhaps in the back of my mind I was hoping to free up some real estate by doing this. I use a protective enclosure around the plumbing to keep the turtles and fish out. I wouldn't need that enclosure on the side with just a return, freeing up some space. Now that I have experimented with a single return (by capping off the other return temporarily), I realize that my rather large (24 sqft) tank will probably need both returns. So now my attention will turn to bringing the returns over the rim of the tank and having only a single standpipe in each corner. That will allow me to use a smaller protective enclosure around the overflow, giving me more room, or I should say, the turtles more room.
I would never go back to a canister because cleaning the filter media in the sump is a breeze compared to lugging a giant plastic, water-filled container outside to clean. Plus I have an automatic-topoff device that keeps my sump at the same level so I don't have to worry about leaving town for a while. A 24 sqft tank evaporates very fast.
Now to figure out how to secure the return plumbing outside the tank without having a nice bulkhead to secure it.