It's allowed, but why would you do this if someone else had done all the work up until that point? I ask this question from both sides, as I've been the one to stand aside and merely watch if someone else is attacking the mvp, but I've also been the person who's had the mvp stolen from them. Knights like to lure Orc Hero away while I'm setting up the icewall etc, like I said it's allowed but extremely annoying!
I know this happens, so I give a tutt and move on. But there are disgraceful incidences that occur, like when I was killing Dark Lord and a pally, iirc, blatantly ignored that I had a good position and was working on the mvp, and lured him away right across the other side of the map to his waiting party. That kind of behaviour is not necessary, free for all or not.l