As has been said, the critical factors are quantity of developer concentrate and agitation. If you double the quantity of developer concentrate when doing two films and use a tank that gives similar agitation you should be OK.

I use a 3-reel Paterson tank and 500ml of solution for a single 120 roll. Using the 3-reel tank ensures that the developer drains right out of the film with each inversion of the tank. (A 2-reel tank may be adequate, but I had a 3-reel on hand, so I use that.)

This complete draining may not be critical with 120 film. With 35mm, it avoids the the problem of uneven density around the sprocket holes, as discussed in the Darkroom Photography article, and it seems not illogical to apply the same technique to 120. If nothing else, it’s nice to visualise the film inside getting an even dose of fresh developer with each inversion of the tank.

If I were developing two rolls at once (which I don’t), unless the tank is big enough that both rolls drain, I’d be concerned that the two rolls were not getting identical treatment. I might be being unnecessarily sensitive on this point, but I’ve read comments about Rodinal being more sensitive to agitation than some other developers, and experiment seems to bear that out.

I used to use 1+50 Rodinal, with “normal” agitation (30 seconds of gentle inversions, then four inversions each minute) and closely controlled time and temperature, but had trouble with the highlights becoming too dense. Densitometer measurements confirmed that the curve was running away (curving upwards) at the right-hand end. And this result seemed consistent with advice from a mentor to agitate very gently.

I experimented with greater dilutions, less agitation (30 seconds of gentle inversions, then two gentle inversions every three minutes), and longer times (about twice as long as before). From successive experiments, I found I was developing to exhaustion: more time did not increase the contrast further. And the results were what I was seeking: a smooth curve that was straight at the higher densities (did not curve upwards).

With this latter procedure, 1+80 is now my “normal”, and I use greater or lesser dilutions to get more or less contrast. It seems to work well for me. The technique is similar in principle to using two-bath developers.

I’m religious about maintaining the temperature at 20 degrees. But, because I’m developing to exhaustion, I suspect that temperature is less citical, although I haven’t established it experimentally.