Two bishops cannot build a lasting fortress against a queen: the queen attacks them from squares they cannot cover together, drives the king to the edge, and wins a bishop with a fork or a pin.
Unlike rook and minor piece, two bishops usually lose to a queen, although the win can take very long (some positions exceed the fifty-move limit). This drill is a shorter one: the black bishops on e6 and f5 protect each other, the king on f6 covers them, and still White wins in about fifteen moves.