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.