>
> Let's assume we use one shared branch for everyone. Then two cosigners
> could need a new receiving address at the same time, and get the next
> unused address on that branch.
>
This is the part I struggle to understand. There is no shared branch
because each user/cosigner has their own unique seed and thus unique key
hierarchy, right? What you described above could be an issue if all
co-signers shared the same seed but then the scheme wouldn't work.