IF you have a darkroom, you can develop different films in the same tank at the same time, for different developing times. Here is what you do. You fill the tank with developer and leave it in the sink. You then load your film, keeping track of what film is what. When you start, you put the film needing the longest time in the tank, put on the lid, and start the timer..
Lets say you have a film that requires 10 minutes developing and one that requires 8 minutes developing. In the dark, with your glow-in-the-dark timer you watch the time and after 2 minutes you open the tank and plop in the 8 minute film. If then, all the film is in the tank, you can turn on the light and proceed normally.
The problem with that one is that film requires an initial hard agitation to get the air bubbles knocked loose, or it needs a prewet. I've done that, but it can be difficult to achieve good uniformity. The times I've done it were for experiements where only the sensitometric scale was important, not picture quality.
So, for practical purposes, the top reel or last one to go in, gets an initial agitation that is needed and you hope that the one underneath is not over agitated. Myself, I did it with film racks in a large tank and kept the racks widely spaced to prevent this type of problem. If needed, I also held the second film in a prewet for a minute before development.
To avoid over-agitation and over-development of the first roll in, you just cut back a tad on the development of the first roll in, thus to compensate for the next roll in, which also gets a bit more vigorous agitation cycle for the first agitation cycle after you drop the film in.
Or as PE says, you could plop all the reels of film in a water presoak and transfer them as required to the developing tank.
A far better alternative is to use 2 or 3 developing tanks.