I'm not sure if they could pull that off or not. It'd give Sprint a controlling interest in their nearest competitor, and it'd be hard to prove it wasn't an anti-competitive move. Not to mention if they did do that and kept both brands operating they'd essentially be competing with themselves.
It would be as much a Department of Justice issue as it is FCC. Sorry scarbrobill. They can try to proceed by DOJ has hinted they would attempt to block the deal. The could take on the DOJ but it could prove to be a poor fight to pick.