Setanta gets first pick on either Tuesday or Wednesday each week, so in other words TSN will be forced into a second pick one week and Sportsnet will be the next week. It has a lot to do with people needing to subscribe to Setanta in order to make the high price of Champions League profitable. The good news is that the second leg or Arsenal-Barcelona should air on TSN2 (I think TSN has the Brier that week if I'm not mistaken).
For the semi finals last year one leg of each match aired on Setanta, and one aired on Sportsnet IIRC. I think that this year all four semifinal matches will be on either TSN or Sportsnet, but I'm not sure.