Split muscles

Your stomach muscles are adhered to your torso infrastructure-far beneath any fat layers that may exist above. They don't have a lot to do with each other as far as tonality goes until you are talking about true cut-levels of definition.

Do you have diastasis recti? There are specific exercises/movements that can help with that. Here is a page about them: http://breakingmuscle.com/womens-fitness/12-weeks-of-workouts-to-rebuild-after-diastasis-recti.