Last week I managed to load two rolls of 120 onto the same reel for the first time.
Make sure the reel is absolutely dry or the film may jam.
After loading the first roll, carefully peel the film away from the backing paper - it is held there with a small strip of tape.
Use this tape to stick the start of the second roll to the end of the first making sure that the edges line up then carry on loading.
If you have a couple of scrap rolls it is worth doing in daylight so you can see what's going on (I didn't bother doing this though).
I do it regularily. I tear the tape off of the backing paper on the first roll, leaving a bit of tape on the film. I wind the first roll all the way to the bottom of the reel, and just load the next roll after. The tape on the first film seems to stop the second roll from rolling over the first one. However, just to be careful, I ONLY do inversions for agitation, with no swirling action. Works for me.
I did what 'ricksplace' described above this weekend. This is the second time I've done it and so far no problems. And I've used both inversion and swirling agitation.