I managed in Berlin to run uniGUI with extjs 4.2.5 and in Tokyo with version 6.5, but I had to do a lot of things manually.
it would really be great if you had the ExtJS 6.5 version run separately so you could run tests.
including Runtime, so we could have projects running with 4.2.5 and some tests in 6.5
In same delphi, haveto change class name, and all of them haveto change. This is not easy and it does not make sense. For example, if tunilabel changed, class name and unitname changed aswell, you have to change in project all, this mean can not use same time. maybe billion ifdef directive need. My idea is virtualbox is usefull for this, I mean same delphi version different unigui version (sencha 4.5 & 6.5)
But same computer, different delphi version can be, because delphi version use different registry keys and folder. I mean On same computer, Delphi berlin installed and installed in sencha 4.5, this computer has delphi tokyo too, and it has sencha 6.5. ofcourse unigui hasto install different folders.
For virtualbox, I'm not tested, but, you can get image your pc then restore this image in virtualbox.