If Allen goes down, they will sign Tanny or another guy for the rest of the year. Mitch is on the roster, and will be here most likely for the entirety of the year.
Backup QB is an area you dont worry about until that problem is in your face. Allen has shown to be incredibly durable, and you need to bank on that. If he goes down, then you evaluate and make a decision, because Mitch/Tanny aren't bringing this team to the playoffs no matter what.