A. 505
The large creature can still push the medium creature 5 feet back but will be unable to move with the medium creature to push it back further.
You could make house rules for trying to bull rush both creatures at the same time or perhaps overrunning one while bull rushing the other.
Bull Rush is a standard action by itself, so unless the creature somehow could make two standard actions in a round that would not be possible.b) If 2 med creatures are standing together can a large creature make a melee charge attack against 1 and a bullrush charge attack against the other.
No, you cannot normally make an additional attack in the same round you also take a standard action.c) When doing a bullrush as a standard action can you still make a melee attack. (I think the answer is no, but I've never used bullrush so I want to make sure of everything).