A two-move knight is certainly not a Nightrider, but still ... In some cases, in the endgame, promoting a pawn into a knight by the first move, you can immediately hit the opponent's king by the second move.
Nevermind. I was reading the object as capturing two knights instead of two kings. To prevent this misreading, I have revised your sentence to read "To win the game of Double Move Double Chess, a player must capture both of the opponent's kings." This makes it clear that there are only two of them to begin with, which is true for kings but not for knights.
Nevermind. I was reading the object as capturing two knights instead of two kings. To prevent this misreading, I have revised your sentence to read "To win the game of Double Move Double Chess, a player must capture both of the opponent's kings." This makes it clear that there are only two of them to begin with, which is true for kings but not for knights.