Angel was created only two years after buffy had been on and they both ran for about 5 more years. Buffy for 7 seasons total and Angel for 5. I would say Star Trek and to a lesser extent Stargate both fit the concept though as the keep extending the same universe and premise far beyond the initial concept. The reason why you typically don't see spinoffs at the same time is due to production costs and studios focusing on one show, but TWD has become quite a bit larger than any of these shows and will be able to utilize many cost saving measures by running at the same time.
In any case, this is really a unique circumstance and is more of a continuation of the universe much like you have multiple versions of Marvel programs running all over the place. There are solo movies, group movies, TV shows that tie them together, more TV shows coming, etc. That's probably the best example to look to for how this will operate.