A while ago I asked what folks do when they have, say, a 36 exp roll, a 24 exp roll, and a 12 exp roll ready to process, but have only a two-reel tank. I'm not sure what replies there were but I eventually came up with my own solution that I've been using for a while now, and it works great. Staples!
The trick here is to leave the leaders out when you rewind, or extract them with a handy-dandy lead retrieval tool. This allows me to join a couple 12's or a 12 and a 24 and still have a spare reel for loading another roll. They wind fine onto my Patterson plastic reels; not sure how well this technique would work with steel reels.