Here is a short(ish) video tutorial I made for the Film Shooters Collective on how to respool an exposed roll 120 film for film swaps/multiple exposures. I also regularly respool 120 film onto 620 spools for shooting in my Kodak Medalist and other 620 cameras, in which case I just take the unexposed film off of the 120 spool in a changing bag then rewind it onto the 620 spool.