On some markets, they do overlap. It's neither practical, nor does it make sense to have a syndicated host who has his own show fill in for someone else. What does it say for an established (re: nationally syndicated) talk host when he fills in for someone else?
Also, Station "A" that carries Medved would not like it if Station "B" (their competition down the dial) had him on at the same time. They pay good money for Medved's services, why should someone else get him for free?
Look, you can try to spin it any way you want. Medved made it very clear that he wanted to continue filling in for Rush. If it were a syndication/contract thing, he would have known about that. He made it very clear that that was Rush's call.