if you have a stereo pair, I'd say that you have aerial photos, not ortophotos. Ortophotos are created by correcting the terrain distortions on aerial photos.
Creating a DEM from aerial photos is not as simple as we wish. First, remember that only the area where the photos overlap can be reconstructed.
Commercial software like PCI Geomatics or ENVI offer that functionality. Agisoft Photoscan is made for Structure-from-Motion, not traditional photogrammetry, but is very good and not expensive.
pix 4D is a software that is not open-source and I never tested it