Look to me that a twofold rook exists in two kinds:
1- A a1a2 rook that is twofold when moving N-S, but singleton when moving E-W;
2- A a1b1 rook that is twofold when moving E-W, but singleton when moving N-S.
Am I right? Or the twofold rook is a1a2b1b2 piece, like the dev.
Hugs.