Mate is possible in the corners and edges. Move the Bishops to diagonals next to each other. 1. Bb8-f4 already d1-h5 diag is controlled, this move controls c1-h6 diag 1. ...... Kd5-e4 2. Bf4-h6 if Bishop is attacked, go to the other side on the same diag 2. ...... Ke4-e5 3. Bd1-c2 increasing the cutoff to b1-h7 diag 3. ..... Ke5-d5 4. Bh6-g7 increasing the cutoff to a1-h8 diag 4 ..... Kd5-d6 5. Bc2-b3 increasing the cutoff to a2-g8 diag 5. ..... Kd6-c6 6. Bg7-f8 increasing the cutoff to a3-f8 diag 6. ..... Kc6-c7 7. Bb3-a4 increasing the cutoff to a4-e8 diag 7. ..... Kc7-b7 8. Bf8-b4 Kb7-a6 9. Kh1-g2 if you cannot increase the cutoff bring the King closer 9. ..... Ka6-b6 10. Kg2-f3 Kb6-a6 11. Kf3-e4 Ka6-b6 12. Ke4-d5 Kb6-a6 13. Kd5-c6 Ka6-a7 14. Bb4-a5 increasing the cutoff to a5-d8 diag 14. ..... Ka7-a6 15. Ba5-d8 if Bishop is attacked go to the other side of the same diagonal 15. ..... Ka6-a7 16. Ba4-b5 Ka7-b8 17. Bb5-a6 increasing the cutoff to a6-c8 diag 17. ..... Kb8-a7 18. Kc6-b5 Ka7-b8 19. Kb5-b6 leaving the black King with only 2 squares freedom 19. ..... Kb8-a8 20. Ba6-b7+ Ka8-b8 21. Bd8-c7#