Ah, you can do that simply by making a stereo tour in Pano2VR, and using the same image for Left and Right when you want to show a monoscopic panorama.
The Demo tour in the app has a mix of stereo and monoscopic panoramas as well. The 'Villas Zeist' are monoscopic for example, simply using the same image twice.
Note that for VR Tourviewer to recognize the tour as 'stereo' you have to add the User Tag 'stereo' in at least one panorama. That way the fact that the tour is stereoscopic is stored in the XML file. Best practice is to use the first panorama for this.
