If you work on strengthening one muscle too much, you can be in for trouble. You have to a keep a balance. So if you work on abs only and don't do any back strengthening, you can increase the risk of back injuries. So while they don't necessarily need to be done on the same day, you should keep things mixed up over the course of a week. Not sure if that makes sense and other coaches will probably have better input, just what I've heard over the years and my understanding.