Multiple Fuseguard versions for single ColdFusion Instance

Yes, you can. Two things.

First, this is no different than would be true of ANY cfc. While some would rely on cf finding it via a mapping defined in the cf admin, you can instead define the mapping at the application level (in application.cfc or application.cfm). Those could each point to a different path, where the FG cfc would be found, perhaps as different versions. 

In fact, you don't technically even NEED to use a mapping: you could put the FG cfc and supporting files instead in a folder within some given application, relying on your application to then load the cfc from THAT subfolder. Of course, having more than one copy of a cfc is generally to be avoided, which is why instead one typically locates it via a mapping instead. Again, see above.

Consider also that if you use the database configurator feature for FG, you'd want to use a different database for each version during such testing. 

Second, while it's OK to ask here, note that FG is owned by Pete Freitag and his company Foundeo. He might even offer a different/better suggestion.

He offers free direct support to those who license his products. You could use the chat or contact form feature offered at https://foundeo.com/.

Or let us know if the above helps or not, or if you have further questions on it.