That and... if you get two living donors, you have TWO lungs to worry about rejecting. That is to say, if you get two lungs from the same person, you only have one other person's organs to possibly reject and worry about. If you get two living donors, you have two people's organs... and logic suggests, a better chance for rejection.
I don't have a source for this, so don't quote me at all. But if I remember right, the statistics for two living lobe donors are poorer than they are for one dead donor.
...I just read Lauren's response, so chances are, I'm full of sh*t. But I don't see why two donors would be less rejection than just one. Doesn't seem to make sense?