Quoted from silver_spinner:1.6 code: this is with GLASS off.
you can stack snake multiball only with coffin multiball.
you can stack grave marker multiball only with coffin multiball.
you can stack coffin with sparky mball (in that order only)
you can NOT stack snake with sparky, or snake with graveyard or sparky with graveyard.
So 3 MBs stack with Coffin. And can stack FUEL and the METALLICA letters/ramp mode(Whats it call?)