I would put my foot down with both of them straight away, you get on with both of them and thats all that counts, if neither like it well tough luck, point out to whichever one your next with, that your not here to bitch behind the other ones back and you won't have your friends slacked off in front of you, if they don't like each other their is nothing you can really do, point out if they carry on slagging of each other to you, they will more than likely have one less friend in future?
Sometimes you have to be really up front with people, these two deserve this treatment.