This is coming from someone who has become a big fan of Portable Apps. What you're doing here is terrific stuff. In fact, it is so good that I've developed my own method of running software in a portable fashion from the same thumb drive as what is used for real Portable Apps. My method is somewhat crude when compared to the way that portable apps work. While not having any real in depth knowledge of what's involved in making a program, that might normally be installed, portable, I'm think that one such consideration has to involve handling the Windows registry in a way that its' integrity is preserved. My technique does NOT do that but still seems to allow me to switch between different computers of mine and should probably not be to harmful to other computers where that particular software is not used.
However, my technique does do something that I think is particularly valuable that does not seem to be offered by Portable Apps. When I migrate to a new version of a program the process does nothing to prevent continued use of the older version/s. This can be very desirable. Especially for those of us who are somewhat anxious to try the latest and greatest.
With that said I'm inclined to believe that the very things Portable Apps must be doing to make a program portable are the things that cause installers to prefer replacing older versions. My guess is that the registry is one of the, possibly the only, real issue and that Portable Apps have solved the problem.
My desire to have multiple versions of an app available for use does not depend on being able to run multiple versions simultaneously. Hopefully this explanation is sufficient to understand what is being suggested and therefore explains the question being asked.