Oh ouch. Uhhh im not sure you should be doing ab workouts if your muscles have separated, doing really any ab workouts could make it worse. As to ab workout work more on a shearing motion. Talk to your doctor. Only things i ca n think of thats light enough for something like that would be Very light torso twists ( i mean very light) slow slow motions NOT twisting as far as you can. You could also (if you can) bend over and touch your toes, come back up and give a small flex at the top of the rep..... man be safe i dont want you to get hurt more
I would stay doing long planks for now
You really should consult with a physical therapist before initiating any core workout program.