It depends on why they aren't getting along. A lot of times, SOMEONE is overstepping their boundaries, whether it be the spouse or the parent(s). Combine this with a grown child who refuses to put their foot down, and a disaster is born.
I do know a few friends and seen some dramaful stories on LHCF. However, in each and every example the man was too weak to stand up to Mummy dearest which allowed the situation to continue. -_-
I remember talking to my ex about meeting his parents and he said if they disrespected me, or turned out to have racist tendencies he would cut them off. That was the end of that and he was dead serious knowing him lol.
Thats a bit drastic but personally if the two couldn't get along despite me putting my foot down I wouldn't have them around each other. I also would cut off any conversation from my family talking about my partner. Just nothing, no continual dialogue, no family dinners etc.. unless they agreed to be socially polite.
My grandma acted "off" towards the bf I introduced her to, so I don't mix her with my partners.