I don't think it's normally possible--when the muscles relax to empty one's bowels, the urinary channels open as well, meaning your waste fluids would release. I'd say that it could be done, if you absolutely had to hold it, but that sounds like it would put more strain on the area, because you're willingly tensing up the groinal muscles and channels while applying force with the bowels. That or you have a high level of anxiety, which means you need to learn to take a good poo anyway, something that, scientifically proven, helps people calm down.
Sounds painful, really. Beyond that, why would you want to be uncomfortable on the john? That's a short period of relaxation, where you're alone to do whatever you need to do, and by gods, you should be relaxed both during and after the act.
