If Firefox portable is closed and I open a cmd.exe window (or run a batch file) with something like this: FirefoxPortable.exe www.mozilla.com
there won't be any problem.
If I have a Firefox portable window opened and try to run a Firefox Nightly portable window it will reply with: "another instance of Firefox is already running".
Ok. If I edit the FirefoxPortable.ini of the Nightly edition and set AllowMultipleInstances=true it will allow me to run the browser but no URL will open through command line on that browser, complaining that: "Firefox is already running, but is not responding".
This happens on current Firefox too.
So, running two different Firefox editions can be achieved with AllowMultipleInstances=true but this disables any command line support for opening URLs.
I've been trying different things about it for some weeks as the "-no-remote" parameter (on AdditionalParameters=) that I've seen suggested, but looks like this doesn't affect the problem.