You don't stuff/force the second feeder into the snake. You just sneak it's head into the snake's mouth as the first feeder is disappearing and the snake's "chewing" action should cause it to grab and swallow the second feeder.
Is he saying he left the second mouse in the cage by its self with the snake ? that could end up going the other way espcially if he just ate .. but glade it didnt ugh (lucky)