One return won't work though, because of some of the points already made.
I'm pretty sure back pressure is what everyone is getting at, the pumps fighting each other, the strongest will obviously always come out on top, ultimately leading to damage of one or both pumps. I'm not sure if they're even the same type pump/filter he wants to connect, but one will no doubt be slightly stronger even if they're the same.
Going along with James D original plan and knowing one return isn't possible, only having one outlet obviously reduces any visible pipe work by one, if it works that is. It's one less anyway
If you have one pipe with a bigger diameter coming from the tank and then split to the filters, I can't see why this wouldn't work. Maybe even same diameter would work. There's a lot of siphon/gravity involved which should be able to supply both pumps with the required water. They wouldn't be fighting each other as long as the required water for each pump is being fed. The fighting comes after the pump.
You could have 2 pumps needing a 60/40 split of what ever water is required, or 3 pumps requiring a 10/40/50 split of what is required etc etc As long as 100% is being supplied (adequate pipework) and they're all running an outlet each, I can't see an issue.
That's my thinking anyway