Thanks for this tip! The first time I tried this I thought nothing happened but I must have been too impatient as when I tried it a second time the new instance did indeed appear. I completely quit Joplin and restarted and tried again. After the normal (for v3.3) 8 second startup delay I went for the "new instance" and after a delay it appears.
Comes up with the standard "first time" welcome screen with instructions and there are no notes or profiles but a second instance is there. Just have to figure out what to do with this new installation.
In Windows the new installation creates "C:\Users\username\.config\joplin-desktop-alt1" and nothing of the original installation is available to it. It could be thought of as being able to have two "profiles" active at the same time but with completely different configurations.
As well, it's a toss-up once you close one of the instances as to whether you can get it running again. I have found it best that if I want to close one, I close both completely and start over. After playing around for an hour or so I think I'll stick with the single instance with multiple profiles.