I think the best way is to think their way. you should realize the necessity of their getting separated.
I don't want to be philosophical.. but there is a saying
"It's better to leave the shattered pieces of a glass the way it is,to fix it again would only hurt and tear you ."
Some relationships are better if they are left the way they are. If you try to make up the things again it would only hurt you just the way a shattered glass piece does.
Again, it's their life . You should let them go the way they want.You should carry on with your own life..
If you can't make the things perfect b/w your parents parents . At least try to to make them decent enough for the future!