"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.