Did anyone notice in the episode "Scared Straight" that when McVicker looks at the yearbook, the pictures of Beavis and Butt-Head are right beside each other? (I'm sure MJ drew it this way just out of expedience) Now this raises the eternal question of their names yet again. If they had the same last names--which would kinda make sense if they might've had the same father and took his last name--Butt-Head's first name would have to come alphabetically before Beavis' since he's to Beavis' left. (Also this would mean that "Richard" Head would be Beavis' cousin as well) If they don't have the same last names, then at least we can surmise that their last names must be close together alphabetically.
Of course, we'll probably never know the answers to such profound mysteries, but this is the kinda stuff newsgroups were made for.