When two bishops are alive, the side who had them would had better chance to win than two knights. Two bishops controls two color of diagonal, and even though one of them could be locked up, however, the other one is still active. And the bishop can transfer from one side to another. The short ranged knight cannot!
It is always hard to decide whether knight is better than bishop or bishop is better than a knight. I think it is going to be in this formula
Lone Knight>Lone Bishop
Bishop+ knight> Knight + knight