@rkagerer Yes, you can run both PreForm versions side by side.
On Mac, just rename the application in the Applications folder with the version number in the name, and then install the 2nd one and rename it with its version number too. They will run at the same time without issue, as well.
On Windows, choose a different install directory during the install process (personally I add the version number to the Preform folder at the end of the default install path, so one folder would be “PreForm 2.20”). They will run fine one at a time, but it is a little tricky with Windows to run the two versions at the same time. You have to run one as a different Windows user, you can find instructions for this here: https://www.makeuseof.com/tag/how-to-run-multiple-versions-of-the-same-program-on-your-pc/