create a new folder and install it to that
call it Carrara beta
you can reinstall copies of your plugins to that folder too
you manually create a shortcut to the program or your C8.1 dep on which one the desktop shortcut goes to
I actually replaced C8.1 64bit but reinstalled C8.1 64bit to my external drive if I really need it, I have C8.1 32bit and C8.5 beta 64bit on my computer.
stuff done in the 32bit renders in the 64 so if I do not want to use the beta, I just do it in the 32bit save and then render in tge 64bit beta.