Multiple accounts on the same server is common, eg for people with more than one phone number from jmp.chat
I'm sure it happens, but I imagine it happens considerably less than a single account talking to multiple people.
You need protocol to have bare JID bound to the quic connection anyway, so we can have both. Though the only thing spec'd and implemented now is each stream is independent which doesn't need protocol.
As noted elsewhere, I've implemented a PoC for multi-stream QUIC, and a PR against XEP-0467 for it. Would appreciate your comments/feedback.
I would note that I'm actually running discovery via SRV records; I'm not keen on the idea of using an HTTP call to get the connection details, and haven't revisited SVCB for XMPP yet - though I probably should. I've deliberately not included this SRV discovery in the spec, I think that's entirely orthogonal.
Dave.