Up near the top, I suggested splitting the XP or giving full XP to both if you are generous. In that suggestion, I was presuming that the second character didn't know that the creature/foe was dying. If he did know, and the player WAS trying to do the first player/character out of the kill points, then I have to agree with those who say that the second character should get no XP out of the situation.
Like a number of others, I also do not use the core XP system from Rolemaster, as that system, to me, requires too much tracking and promotes combat and spell use over other RPG activities (it also encourages min-maxing to get every possible XP out of an encounter). In fact, I don't think that I have ever used it. In the past, I have used a Daily Activity Level XP system from one of the Companions for a long while.
However, I prefer Goal Oriented XP systems, like the one in HARP and also presented in RMX. Using that method, you actually reward players for showing initiative and for getting things done. The more they accomplish, the more they get.