Since there is no pressure and relieving yourself in the vacuum of space might cause an implosion in your system, it might be safe to say that the farts just stick to the poop the way liquid sticks to things up there. You don't ever actually accumulate the pressure necessary to evacuate your bowels so you just wait til it fills up and slowly builds up and out of your asshole. Like that fabled pile of trash that eventually sticks out the ozone layer and "erupts" out of the atmosphere.
