For my equipment it certainly IS true. I don't get anywhere near 8-10 cm between films. If two 120's are loaded edge-to-edge in a Paterson reel, then the tip of the outer film barely manages to squeeze inside the ball bearing. It is a very tight fit. Rotary agitation causes the film to slide more easily in the grooves, and allows the two films to overlap. It is not only a development problem, but also for the sake of proper washing. I had a case where the films overlapped after I rotated the spool during fixing. Subsequently I could not get the fixer washed out properly. I had to take the films out while wet, rinse them while handholding, then dry and respool them to be sure I got all the fixer out. I was very annoyed with myself, but I learnt the lesson. I would have fewer qualms about multi-reel developing if I did very frequent developing. But since my chemicals tend to stand for a month or two, sometimes longer, in between developing sessions, I feel that my risk is best spread by handling smaller batches.
You should get 2 test rolls and load them in the light to see how this works. There is room on the reel for a 220 roll + plenty of margin, thus there is also room for 2X120 + some space in between. There might be less then 8cm, but plenty of space, still.
Two comments about paterson - the twirl agitation (which is not rotation) went out when cars had fins, and - ball bearings on the reels are a real impediment when loading non perforated film.
If you want to rotate, rotate the entire tank, otherwise, inversion is probably better then the twirl stick (and will help keep films in the same place you left them when loading).
Jobo reels, do not have any of the above mention problems (with or with out the use of the duo clip), the tanks use less chemistry and they do not leak.
All that said, stick with what works for you, or upgrade if you want more capacity with less worry.