I guess what I don't understand is that it will not replace the icon and just update to MC20. I can technically have two versions.
Yes. Media Center has long (on Windows) let you have multiple different major versions installed simultaneously. You generally shouldn't
run them at the same time (use one or the other), but you can have them installed separately.
This is useful, especially in the "early phases" of each major release (like right now), because you can test out the new version without relying on it full time for all of your needs. Early versions are sometimes unstable (for a few weeks, usually, after the release of big, new features and versions).
This is common for many applications on the Mac, especially "pro Apps". For example, installing Adobe Creative Suite 5 explicitly does not uninstall CS3. Final Cut Pro has traditionally kept the old version when you install the new one (especially with the transition to FCPX, but even before that, back through FCP4 at least, if I remember correctly). Even Microsoft Office works this way on the Mac.