My MIL & FIL sold their home. They split the proceeds. If one passes, does the other gain total proceeds?

Slalom, no one can even try to provide you with succinct answers w/o reading any wills or the trusts they've created.    You wrote "they have a trust fund."  If this refers to your parents, presumably they would have separate wills, and also presumably, the terms of those wills would "pour over" into the trust, or more appropriate yet would be separate trusts.

If you mean the FIL's kids are provided for in a trust fund, again, there's no way anyone could accurately provide the information you seek.   An attorney would have to review the terms of the trust(s) and pourover will(s).

I would think that given the separate bequests, the trusts would have been separate, or at least separate and distinct provisions for their respective children.  

The friction existing between families would be a concern.   These are definitely attorney questions.    And as a potential beneficiary, you should try to first discuss this with your parents.

Hopefully their proceeds were put in separate trusts or the trust has clear instructions for what is to go to whom upon death of one of them.

Can MIL move to a different apartment in the AL? Don't allow AL to tell them her new room number? Or keep her door locked and do not let them in? Call a nurse for help if they are bothering her. Get a restraining order if there is any proof of abuse? Just brainstorming, not sure what is possible. OR maybe MIL should move to a different facility entirely. That would make her day-to-day life much less stressful.

Regardless of who inherits what, you need to record this abuse, contact APS and the police and keep this trash away from this woman.

I don't care what she has done, nobody and I mean nobody deserves to be abused.

I would also report this AL, I know for a fact they are mandated reporters and they have failed to do their legally required duty.

Get the proof and protect this woman now, if you don't, you are as guilty as the trash that is abusing her.

I hope they both leave all their money to charity. Nobody in this family deserves a penny for letting "adults" abuse a senior.

I would check with an attorney but logic says that if they are still married anything they have would pass to the spouse.
If each though as the proceeds in a trust or other holding that lists beneficiaries the beneficiaries would get what they are due.