Keys "s"=save "l"=load "b"=back once
After a legal move on one board, any unit may transfer to the same square on the other board so long as it is empty. I didn't allow castling to do this.
A piece, but not a pawn, may instead of making an orthochess move transfer to the same square on the other board so long as it is not occupied by a friendly unit. If it is occupied by an enemy unit, that unit is captured.
These are simple illustrations rather than strong opponents.
Bug reports? -- Thank you! Keep them coming!