I'm not in the immediate situation but do understand since I have similar issues myself.
For the JW they will stay because of their obligation to the watchtower society and Jehovah and of course the family itself. If its the man who is the JW he may loose his prominanace in the congregation as well.
For the non-Jw partner, they don't have the support of the WTS unless they decide to join - but if you analyse your situation carefully it may be due to your own co-dependancy, or fear of not finding someone better, or simply apathy - it's all too hard to leave.
The situation would be no different if one partner was addicted to alchohol, or had a terminal condition, or belonged to any other fanatical religous cult.
Being in a dysfunctional relationship soley for the kids is tanatmount to martomdom and suffering of personal abuse of one's self.
If your not happy, I can't see that the rest of the family would be happy, as feelings can be contagious.