Something like two mirrors facing each other??
I think they would either react by a) essentially have telepathy with each other since they would be able to just talk back and forth b) only hear static or silence (either of which would probably be pretty creepy for someone who's used to being able to read other people's minds) or c) screaming. both of their brains just completely reject that theirs another mindreader and react by giving themselves a breakdown.
I think they may just hear the others thoughts like "I am reading this person's mind."
Or of any of the above.
If you had two mind readers try to read the other's mind at the same time, what would they see?