My son was like that, pooping on the potty but peeing in his pull-up, so we took away the pull-ups and put him in big boy undies. The first day was awful, he had 13 accidence and was begging for his pull-up, but that was his way of trying to insert control. The next day, when he realized that the pull-ups were not coming back, he only had 2 accidence, and then the next day one, and then none. They always say to wait to train until they show they are ready, well pooping in the potty shows that they are very ready, and just need a push.