There are several factors that make the 2Bs an advantage, as long as your opponent has only one B or no Bs. And, the pawn structure is not closed and your opponent has 1 or 2 Ns.
The factors are:
1.Bs can gain or lose a tempo (even when reflecting off the edge ofthe board. Ns CANNOT. This is especially important when your opponent is in zugswang. Big advantage for the Bs
2.Bs can get across the board in 1 move. Ns take from 4-5 moves to get across the board. Big time advantage for the Bs
3.A Bishop alone can trap a N on the edge of the board. A Knight CANNOT trap a B on he edge of the board. Big advantage to B
4.Bs can hem in enemy pawns and Ns. Ns CANNOT hem in enemy pawns or Bs. Big advantage to Bs
5.2Bs are a mating force. 2Ns ARE NOT a mating force. Big advantabge to Bs
There are more but I think you get the idea.
"They [Steinitz and his successors] also considered the possession of the two bishops as a real factor in guaranteeing an overall advantage, though this particular question, more than any other, is today a controversial one. For example, I can think of few cases where the two bishops proved a decisive factor. As a rule endings won with the help of the two bishops had other favourable factors for the winning side (better king position, greater space, the presence of enemy weaknesses). On the other hand, I can recall quite a lot of cases where two knights showed the greater energy and really trounced the bishops. Hence I feel it would be better for us not to consider the two bishops as a factor that works independently of other factors." -- A. Kotov, from Think like a Grandmaster
Thoughts?