Currently in the relationship editor, you can "batch edit" by "adding" a duplicate AR which then gets merged.

However, this is not obvious at all. As a start, making the link say "batch-add/edit" instead of just "batch-add" would probably help (phrasing that will be hard though)

Also, in order to do this sort of batch editing, you have to completely re-enter the AR, it doesn't get pre-filled with the existing value. Some way to actually batch-edit the existing ARs would be nice.