The UI combines it kinda okay, I think a few people had issues and had to relogin or clean cache, but I'm indeed proud of the cross-posting / cross-voting / cross-commenting feature.
If you check the json data inside DTube videos now, you will see a 'refs' field which basically references on which other chains you can also find this content. As you can see, it's an array and it would be trivial to extend it to any other chain able to store JSON contents as a tree.