Standard Chess, except that each cram square (i.e; a1/A1 to h8/H8) accepts two playing pieces.
Object: checkmate either of the opponent's Kings by attacking it so it cannot escape.
So named for the saying 'capturing Two Birds with one stone'.
This zrf follows a Cram Reversi and Cram Checkers game, developed in 2015.

Two Birds Cram Chess possesses three additional features from standard chess.

First,
Each square can host two pieces at a time (a4, is really a4 and A4, for example).
Enemy pieces may both land upon the same 'shared' cram square (a4 and A4) at the same time.
When capturing, a sliding or shifting piece must move along a clear path to the enemy piece (of either half-square). Note that Pawns cannot simply move north and capture at that adjoining half-square, they must actually move to a diagonal Cram Square (i.e; a4/A4 from b3/B3, not a3/A3) location - at either half-square.
To switch half squares, however, a player's piece will shift over and then perform its typical move.
No capturing, check, nor checkmate, can occur within a single cram square (a4 cannot capture at A4).
Be careful of abandoning your own King into a half-squares' routed checkmate position when making a move or capture.

Second,
When a capture is made, that player receives a second capture of any selected identical enemy piece.
To allow this, a 2nd turn-order per side is involved. When there isn't a captured enemy, then a forced pass-turn is taken.

Thirdly,
There are twice as many starting pieces to accompany the doubled board squares.

Other rule adjustments:
Multiple forms of Castling and En-Passant work in the same manner as all other 'cram square' routed moves.
The castling moves are prevented by possible enemy checks as ONLY upon its own 'HALF' of the cram squares.
Final row Pawn promotions remain as singular moves - AND without a second capture.