It’s not a trivial feature at all, it requires a lot of work and rebuilding of things. Here’s what Zac (iOS dev) said about this a few weeks ago:
I’m planning on taking a stab at it in the near future, but it’s going to be a pretty substantial change. We need a lot of new UI (how do you switch between instances?), error handling (what happens if a webhook to 1 fails, but the other is fine?), storage (how do things like notification categories work when multiple servers can define it? what if there’s conflicts?), etc.
There’s an existing thread in the WTH forum that you can add your vote to.