Buffy the Vampire Slayer and Supernatural
They each spend the first half of their show fighting creatures they know they need to take out, but don't recognize. At the end of Act Two the two groups meet. They spend Act Three in one long "But that's our job!", and finally finish each show with their half of taking out the big bad. All from the perspective of their respective shows, and universes.
They each spend the first half of their show fighting creatures they know they need to take out, but don't recognize. At the end of Act Two the two groups meet. They spend Act Three in one long "But that's our job!", and finally finish each show with their half of taking out the big bad. All from the perspective of their respective shows, and universes.

They trace their lineage using self-tests popular these days, which leads them to them taking a trip to Downton Abbey, only to realize that they're related to the butler.