I have freaky control of my muscles. I drink a lot of water and pee frequently, but can easily isolate the pee muscles from the poop muscles. I poop only when I choose to. It's still more frequently than "normal" people, but it's not every time I pee.
It took me a while to mentally isolate the muscles that do this and separate them, but it's kind of like Magic Eye - once you finally see how to do it, doing it again and again is not a huge deal. At least for me.