I would expect it to fail.
The only setup with two fullscreen displays that makes any sense to me is having two (or more) physical screens. If I wanted to change the resolution of the display, there's al_resize_display().
What if you want to change the depth-buffer resolution or the type of super-sampling or anything like that?
I've never tried, but that may be consistent with how things work on OS X, not sure. Not sure about Windows either. Either way, the behaviour should probably be the same across different platforms.