Posit repository A which is at schema state S(A) = N. Developer X branches it to make B and adds a new database migration, increasing S(B) to N+1. Developer Y

Posit repository A which is at schema state S(A) = N. Developer X branches it to make B and adds a new database migration, increasing S(B) to N+1. Developer Y branches it to make C and adds a new database migration, increasing S(C) to N+1. Now, merge B and C. It's not necessary that schema versions be that rigid. All you really need is that they be monotonically increasing; a timestamp in UTC is sufficient for those purposes, although technically you could run into a situation where a UUID is necessary.