"I would expect that if you are working close to capacity, the longest developed film might use an unequal share of the developer's activity, before the other film(s) is/are added."
Matt--You are probably correct. I think I avoid that most of that problem by starting with all the films and then removing early any needing a shorter time. For example, if one film needs seven minutes and the other needs nine minutes, I suspect that, with most of the development is complete after seven minutes, any extra pep available to the nine-minute film probably makes little difference. At least I've never encountered a problem. Obviously, there could be a difference if the developing times vary widely, say one film at five minutes and another at fifteen. With that great a discrepancy, I'd probably develop separately anyway, since the time working in complete darkness would be considerable.
I've used this method for years,with good results.
Assume you have 3 emulsions :A ; needs 14min, B : needs 12min ,C:needs 8 min.
Prepare chemistry for 3 films.
Load 3 reels and put B & C films in a spare tank.
Start process with film A.
At minute 2, add film B.
At minute 6 , add film C.
By minute 14 you will have 3 films correctly developed,and they can all be stopped and fixed together.
Obviously you need a darkroom to use this method..
that's a lot of fiddling. if i want to develop films of different speeds and/or types together, i'll just use a two-bath developer. from asa 25 to 400, it all works well because development time is not a variable. Anchell/Troop TD-200 or divided D-76 is what i use. you can't push or pull the film, and you get some highlight compensation using this method, but you get proper film speed (aka shadow development). i've done Adox CHM 400 and Ilford Pan F+ in the same tank.