The thing is he needs to make 4 rolls total, and each of them takes a full round action (6 seconds). And he has to free all 3 of the manacles for both of them to be able to move (thanks to how they are attached). And then, after getting the manacles off, open the cell door. The 2 guards and Davhid would have ample warning time to shoot them with arrows (and Davhid is there to do just that if they do try to escape).