Ok, let's try for focus.
While it is technically the case that a cock in a mouth and another in a nether opening qualifies as DP, my understanding based on way too much consumption of porn and observation in forums like this is that DP means one cock in the pussy and another up the butt. So that's how I'm, um, approaching this subject.
Based on personal experience: several ladies on the board (currently active and otherwise, mostly named) have engaged in DP; based on my personal experience it is difficult to do; the difficulty stems in part because the concern over MM incidental contact is a deflating factor to many guys and; it's hard (pun intended) to get two cocks into the same woman at the same time. Size does matter here.
All that said, when it works it's awesome. And 'awesome' is a word I do not use lightly. For her, when it works--and she isn't simply doing it to earn money--she is in ecstasy (which is the ultimate turn on for me.) And I have to say that I groove when another dick is working alongside mine to drive her crazy.