I'm pretty sure he works for TSN primarily, but was hired on for NHL on NBC as well.
Well he does, but I think the OP is wondering from a contractual perspective. Normally you wouldn't be allowed - if under contract to TSN - to work for a rival. Now there might be an exception here since TSN uses NBC's feed to show the games, but generally it's a breach of a non-competition clause.
So this leads me to think that either there is an exception as mentioned above, or he is in someway a free lancer with some special arrangement.
I've actually wondered the same thing for a while, I think that has to be the case. I know in the US they like more colour than play by play and McGuire provides that, I think that goes hand in hand with the passion for football, Pierre gets to chime in with his 2 cents on NBC.
I am not sure why we have this thread, it should be obvious not every contract is created equal, but Mcguire probably has a contract to do specific games for TSN that are identified well in advance. Aside from that he can do whatever work he wishes in radio and other TV telecasts, as long as he meets his obligations in each of his contracts and they don't collide.
As for competing networks, that may (or may not) be in the contract, but remember that in baseball guys like Pat Tabler or Rance Mulliniks will do a Jays broadcast on Rogers Sportsnet and then will be doing the same role on TSN the day after. As long as the contracts sort that out there's little problem.