I like a guitar in Irish music but I believe the instrument poses the same problems as the bodhran and one of its own so I can understand suggestions towards restricting the number of guitars.

The first one is the problem of rythm. A good guitar player can often take a strong role in this department and melody players can take a lead from it. Even if we are talking about 2 or more reasonable players, settling the music into a grove becomes more difficult if 2 or more rythm players impose thier own slightly different interpretations and playing styles.

The second is the choice of chords. Again, assuming good players, while either of 2 players may come up with a nice set of chords that enhance the music, the 2 sets of chords together can clash.

I think you have to take things like this on the merits of the individual session and the players present rather than have hard and fast rules about numbers. Having more than one may or may not work and if it works for your session, enjoy it

Jon