I wouldn't have faulted you for going with Staley. If there's a drawback to using Bettis in that scenario, it's that he needs to line up a bit further back to get momentum, which could telegraph the play. Ironically, it's that momentum that's so valuable down there by the goal line.
If I had that team, I'd put them on the field together more often to prevent LBs from keying on one or the other. But I also couldn't have coached 'em to 16-2, so Cowher obviously was doing something right.
Yeah, I'm really fond of a pro-formation (also known as a splitback or Y formation, with 1 RB to either side of the QB at almost HB depth) when you've got two solid running threats, with one on each side the opposing defense has a hard time knowing who to key off on, great formation to pass from.