Keys "s"=save "l"=load "b"=back once
Captin George Hope Verney's Four-Handed Chess is one of the most popular forms.
Win by having both your opponents in checkmate at the same time. You may indeed capture your partner's units. Pieces belonging to a player in checkmate cannot be taken and are considered not to give check. I think that the latter can cause some problems in the program but these should be rare. If your partner is not in check, your move must not place your partner in check. If your move removes an opponent from checkmate and thus un-freezes units which now attack your king, your king may not be captured as a result. Friendly forces do not give check (error corrected June 11, 2002, sorry). If you are in checkmate or otherwise cannot move, then your turn passes. Pawns can leap a friendly pawn one square ahead. Today the Hughes modification is standard.
Taurus Gaming produces a handsome-looking, reasonably-priced playing mat, and a recommended rule change: If you are checkmated on your turn, your army disappears and the opponents then alternate with your partner move for move.
These are simple illustrations rather than strong opponents.
Bug reports? -- Thank you! Keep them coming!