Hi Raymond,
Absolutely.Ê This is the intention.Ê I have dozens of applications running on a single dataface installation.Ê It makes it easy for upgrading because upgrading Dataface once, upgrades all of the applications.
You don't need to worry about the scope of objects like the AuthenticationTool (which is indeed a singleton object - as is the Dataface_Application object and many more) because PHP applications do not persist between requests.Ê In other words a Singleton object is only maintained for the scope of a single request.Ê Multiple applications will not conflict.
The only bit of advice that will be different for running multiple sites vs running a single site is that it is very advisable for each application to have its own templates and templates_c directories
One other thing to note (and this is not Dataface specific) is that running multiple sites on the same domain will result in common session data.Ê This means that if you log into one site and then navigate to another, it may still have you "logged in" on the other site.Ê This isn't cause for too much concern though, because if the user that you are logged in as doesn't have any special permissions for the other application, he still won't be able to do anything.Ê I.e. this isn't anything to worry about.
Hope this helps a little.
-Steve