Ways to Detect Duple-Checkmate but not Allow Directly Capturing a Pseudo-Royal Piece

[PraseodymiumSpike]

Discussed in #819

^{Originally posted by PraseodymiumSpike August 13, 2024}
Is there any way to detect duple-checkmate but not allow directly capturing a pseudo-royal piece? I wanted to make a game similar to Spartan Chess, but where the chess side has to win by duple-checkmate and can't just capture 1 royal piece and checkmate the other. To detect duple-checkmate, extinction is required, but the only way to not allow capturing just 1 royal piece is to make the opponent lose when they capture it, which also requires extinction! Furthermore, these 2 uses conflict! I tried to just make it so the number of pieces required for extinction to be declared is 1 meaning that when 1 pseudo-royal piece is taken, the taker will lose, but that means check triggers when either pseudo-royal piece is attacked because the second-to-last royal piece is considered the last one before extinction is declared. Now, I tried other ways to detect when a piece count drops below 2, but none worked. Connection lets you detect when a piece is on the board, but it doesn't let you detect when a piece is captured. The same is true of capture-the-flag rules. I even tried making pieces go to hand and forcing the pseudo-royal piece to be dropped whenever possible, then making it impossible to drop them anywhere, hoping to cause a stalemate, and then, I made stalemate a loss. But it doesn't force you to drop when there is nowhere to drop, so that didn't work either! Is there any way to make this work?

[PraseodymiumSpike]

It is similar similar to #606, though I assume iron pieces would not be subject to check or checkmate.