Hmm.. then an interesting question would be:
If say, one warmachine has been destroyed, and two of the crew are still alive. Now you have another warmachine standing around of which only one crewmember lives. As you just stated, the two crewmen from the first warmachine can only recrew the other of it's former crew has been entirely slain. But, can the single crewman voluntarily leave his warmachine, so that the other two can take over?