I was thinking the same thing. Tried to figure it out on my own, but got stuck.

Then I heard that it is possible that a threshold musig2 wallet in its current state requires some interactivity between signers, so that you would need each key to sign the psbt twice.

I'm also a little fuzzy on this, but I think what it means is that there's a bit of work to be done before we can all convert our multisigs to fancy taproot addresses that look just like any other singlesig.