Duo backfield is a waste of time. Only one can carry the ball. The other one needs to block. Who would you rather have block- McCoy, Bush or Felton? Or even better, another lineman or TE? And if you say they're going to throw it then why not put a WR on the field? I doubt that Bush is a better WR than Robert Woods. I know McCoy isn't. There's a reason that the two headed backfields aren't used anymore in the NFL.
This is total speculation, but I suspect that Bush is looking for somewhere where he will have more of an opportunity than in Buffalo. Here, he's basically a punt returner and third string RB. I think that there are places where he could be a clear #2 and maybe even a part time #1.