The answer to the underlined question is yes. I've done it before in multiplayer. Just make sure you are allied in the same war.
I did have an issue when an ally of mine's Vassal occupied an elector. since I was not directly allied with the vassal, the elector which was occupied was still counted as independent.
I did have an issue when an ally of mine's Vassal occupied an elector. since I was not directly allied with the vassal, the elector which was occupied was still counted as independent.