Dan Knight wrote an article for lowendmac.com back in 2001 entitled System 6 or System 7? In the article he describes several ways to switch between using System versions on the same Macintosh, and recommends a utility called System Picker:
System picker is a freeware utility that lets you pick which one of two or more System Folders on your hard drive will be active at the next startup. Kevin Aitken’s little utility does this by “blessing” the system you select. (Blessing is Apple’s name for selecting one System Folder as the active one.)
Unfortunately, the link to download System Picker from Apple’s Developer site no longer works. I was able to locate a copy after a good bit of hunting, and that experience led me to decide that I would begin to collect and archive legacy Mac tools to save myself and others the time and frustration in the future. Until that archive is established you can find System Picker here.
I do not yet have personal experience with System Picker but plan to test it soon. Be sure to read Dan’s excellent article for tips on how to install two different operating system versions on the same drive. Not sure which versions of the OS your Mac supports? See this article to find out!