some folks just get annoyed when they see people riding two-wide. I don't even feel safe walking two-wide on our local rec. trail because, ha!, roadies like to blaze down this thing at 30 mph. Back to riding; my girlfriend and I have had a discussion that if we are riding two-wide and someone aproaches, she always goes first and I drop back so there isn't any last second confusion. It seems like a petty conversation to have to have but it helps us be able to ride two-wide (when safe) without feeling like trail hogs.
This is the best idea, since people are never sure of your ability to hold a straight line. It isn't an insult, but on a MUT it is best to single up when approaching folks coming from the opposite direction. You can't tell if they have to avoid, say, blackberry vines growing out onto the trail or a section of broken pavement. Singling up at least shows that you are giving them as much space as possible.