Running Firefox Portable stable and Firefox Portable beta simultaneously

I want to run the stable and beta versions of Firefox portable together. I'm using, and liking, the Beta, but there's a website or two I use that don't work on the beta so I have to use the stable version for them.

The two versions use separate profiles, and I've set AllowMultipleInstances=true in the FirefoxPortable.ini in both. But still, if the beta is running and I try and launch the stable it brings up another windows of the beta. Closing the beta version makes the stable one come up fine (so it's not that I actually have two copies of the beta!)

I tried renaming the beta version's profile directory, and putting the new value in FirefoxPortable.ini and in LastProfileDirectory in FirefoxPortableSettings.ini, but those settings aren't honoured, and upon starting the beta it creates a new profile (named 'profile').

How can I run the stable and beta versions of Firefox together?