Four Kings Variant

In the Four Kings Variation, kings are placed where the queens should be.

All other rules are still the same.

     Yes castling

     Yes en passant

Notes:

Forking or attacking both kings at once is checkmate, unless the opponent can protect both kings at once.

I looked around, and I deduce this is an original idea.

Feel free to give feedback!

Not entirely. In Chu Shogi there is a Drunk Elephant which can promote to Prince, and the Prince counts as a second King. I've also created a few puzzles with a Prince:

https://www.chess.com/forum/view/chess960-chess-variants/chu-shogi?page=2

And you forgot to mention that Chu Shogi was invented some 750 years ago. In Spartan Chess the Spartan army also has two Kings. Both these have extinction royalty, however, where you win by checkmating the last remaining King. (Although in Spartan there is a special 'duple check' rule that allows you to checkmate both Kings at once.) And in the ICC variant TwoKings each side also has two Kings, but which one you have to capture to win depends on their relative location on the board.

The proposed idea has 'absolute royalty', where loss of the first King already loses the game. Such an extra King is a liability as well as an asset. I tested this once, and it turned out the extra King was worth exactly 0. (After deleting the King on d8 in the diagram, white would still score only 50%.) In spartan Chess the extra King is just slightly less valuable than a Rook, the distributed royalty adding about 1.5 Pawn to is=ts tactical value 3.

Actually this exact same variant seems to exist already since 1997:

http://www.chessvariants.com/winning.dir/twokings.html

Pretty cool