How does Counterspying work with multiple spies?
Solution 1:
Do multiple spies provide more defense than one spy?
In a way. It's not a matter of the defense (i.e. catch chance) going up, but rather that the enemy spy has to do two dice rolls, one for each spy.
Which spy gets credit/experience for catching enemy spies?
Whichever spy's roll caught the enemy spy. However, it is unclear to me which spy gets to roll first - it seems plausible that this is based on who was assigned to counterspy first but I have no confirmation on that.