I stumbled across this just recently.
I was in the process of cleaning up my tags, and have decided to reduce "compound tags" as much as possible. So instead of, for example, having a single Tag "Boats; Ships & Underwater Craft" I break that down to "Boats", "Ships" and "Underwater Craft" as 3 separate tags.
Anyway, the "tip", I found that you can right click the Tag and rename it,
but if you just rename in place with commas between the parts, Calibre will split these out into 3 separate Tags (and of course apply all of these to any books that had the original single tag) !
(I thought this was neat)