The relationships from MB are directly turned into tags. In this case the instrument and vocals would always result in two different tags. But the lead and background vocals could be combined in the same relationship on MB, then it would also show up as a single tag in Picard.
There is a plugin to split a single tag with multiple instruments / vocals into separate tags, but no plugin to do the opposite. But would make sense to have this functionality.
This is interesting, it does seem to be possible to edit a relationship so that will store more than one instrument or vocal. Perhaps it’s not supposed to work like this though, see this old bug report: MBS-7677 - Relationships can be edited to have multiple instruments.
But that means that the existing plugin to split combined relationships into separate tags has become mostly useless. And I think we should have an option in Picard to actually combine multiple performer relationships for the same person into one. I added a ticket for this: