A stereo shotgun microphone is just a kind of one-point stereo mike. Specifically it uses M/S, with the shotgun as the "M" microphone. And at low and mid frequencies the shutgun doesn't have any special directional properties, either. Shotguns function as ordinary directional microphones until you get above 2 - 3 kHz.
Above that point, the slotted tube which is in front of the capsule (that's right, the capsule isn't at the tip of the microphone--it's near the back!) begins to set up a somewhat irregular pattern of partial cancellations for high-frequency sound coming from the sides of the microphone. Consonants from speech or singing fall into this frequency range, so this characteristic can be useful for making speech pickup more intelligible when you have to record from a somewhat greater distance than would otherwise be optimal.
That's the situation that film and video sound recordists often find themselves in, since the microphone has to be kept out of the camera frame. However, any knowledgeable film or video sound recordist would instantly set the shotgun aside and use a good supercardioid or cardioid if they could conceal it in the scenery instead; shotguns are troublesome to handle (unless you're in a set scene where the talent always stays on axis) and usually are a distinct compromise as far as sound quality is concerned.
Music recording with shotgun microphones is a last resort since their directional response is so irregular and their frequency response even on axis tends not to be very smooth, except perhaps in the very best of certain models that have come onto the market in recent years (price range ca. $2000 each). For years, film sound was dominated by two particular models that managed to sound both dull and spitty at the same time, and that you had to keep zeroed in on the talent because their off-axis pickup sounded so different from their on-axis pickup.
M/S recording of music with a shotgun for "M" may be only a compromise, but at least it's better than trying to do X/Y or other stereo recording with two shotguns!
--best regards