I can't crap without peeing first. In fact, if I feel the need to pee and to fart at the same time, I can't do it or I'll piss on myself (or I'll just go to the toilets).
Everyone has a time when they're afraid to fart because they think they'll piss themselves. I think either the muscle for clenching your bowls is near to the bladder muscle, or the area of the brain for clenching the bowls is near to the area for urinating.