Hi Jeff, yes it was a problem for me also several years. I think the problem is this: using ordinary small developing tanks and agitating by inversion of the tank, tend to make movements of the developer more vigorous around the edges of the film spiral, thus causing overdevelopment compared to the middle part of the film surface. If you agitate softly (say 1-2 inversions each half or full minute) you may be lucky and for most subjects get an acceptable result, but it's risky, particularly with even surfaces like clear skies and the like, since the exchange of fresh compared with used developing agents at the whole film surface is not optimal. Supposing that I don't have a full guarantee of even development across the whole film surface unless the exchange of fresh compared with used developing agents is optimal, I use the following method. I have always used stainless steal tanks for some reason, I use only one 120 spiral in a tank for 2 and I use only 500 ml developer; I inverse the tank during the first minute quite vigorous, also rotating it in various ways. Now it depends on emulsion developer and temperature how fast the developing process proceeds. I try to keep my total developing time somewhere between 6-10 minutes (or in rare cases even more). Agitating quite vigorously some 10-15 seconds each minute should give you no problems within 6-10 minutes total developing time, but normally I agitate quite vigorously 15-20 seconds each second or third minute (my standard is something like: 18-20C temperature, 8-9 minutes D-76/Rodinal1+50 (TMY-2 Acros 100 i.e. 320/64), inversion first minute, 15-20 sec. the second, forth and sixth minutes.). In my experience with a total development time like 8-10 minutes it doesn't happen very much developing during the agitating period of 10-15 seconds, rather makes the exchange of fresh compared with used developing agents across the film surface very good, which is important. Due to surface tension with a liquid at a film surface, just slowly move the film in the liquid doesn’t make for a good exchange of fresh compared with used developing agents. Following this method/idea for agitation I have no more problem with uneven development from 135 film to 120, and 4x5", 5x7", 8x10" sheet film. I was quite released when I managed to get fully even results also on critical subject like skies and the like.
Using a Peterson type developing tank where the developer goes into the “big mouth” of the tank when you inverse the tank perhaps makes it possible to put 2 120 film in a tank for 1 liter and get good results. Hate uneven development, so I don’t experiment with that! Rather have several tanks for two roll with just 1 roll in each, having cut a piece from a coca-cola bottle to keep the single roll in place in the tank when I inverse it.
Best wishes