If interested, I've a suggestion on a configuration to experiment with-
As discussed above, shotguns do not work well as a stereo pair in standard stereo pair mic'ing configurations that rely on a smooth, well-behaved transition and "hand-off" between the two across the center region of the sound image due to their irregular off-axis response. So instead of trying to use them as a substitute for well-behaved directional mics, consider using one for what they are best at - maximally isolating whatever they are pointed directly at as much as possible, without regards to the quality of pickup of everything else - and use that to supplement a stereo pair of microphones which provide what a pair of shotguns cannot - a smooth and even stereo image with a good sounding ambient pickup.
I'm suggesting a three microphone configuration with a single shotgun in the center pointed directly at the source, similar to the image posted above of the test recording, with the stereo pair routed Left/Right and the shotgun panned to center.
Consider it something of a more optimal division of labor, with each element contributing what it does best and covering for what the other element does not do as well. The smooth spatial output of the stereo pair buries the off-axis issues of the shotgun, and the shotgun improves the reach and center solidity of the stereo pair. To work best, the stereo pair configuration needs to be to modified to accommodate the presence of the shotgun, so don't just add the single forward-pointing shotgun to a typical ORTF or DIN pair of cardioids. The stereo pair should be spaced wider or angled farther apart, or both, to provide sufficient sonic space for the shotgun to solidify and fill the middle of the image. It's something of a close-dance, both supporting and embracing the other, and bit more room is needed to keep the two partners from stepping on each other's toes than if they were simply out there dancing around on their own. The stereo pair, if listened to in isolation, should be on the verge of "too far apart or too wide", and perhaps "too ambient" with a hole in the middle when the center channel with the shotgun is muted.
Record all three mics separately, and adjust the level of the center channel afterwards to dial in the optimal amount of up-front center information in a way somewhat similar to matrixing a SBD feed with a stereo AUD pair. Think in terms of balancing the drier direct sound (primarily from the shotgun) with the wetter ambient stereo sound (the stereo pair). Conceptually it's also sort of like M/S, but instead of a single figure-8 providing the side information you have a spaced pair instead. I strongly suggest recording all three channels separately instead of mixing the three live and recording to 2-channel, because doing so allows for careful adjustment of the center level (and EQ if necessary) afterwards by ear. That center level adjustment is critical, and the ability to dial it in perfectly by ear afterwards is one of the advantages a technique like this can provide.
The interesting question then becomes "what configuration is most appropriate for the stereo pair?", and that is going to depend on the situation. I've not actually done this with a shotgun (I would, but I don't own one), yet I commonly use a single forward facing cardioid, supercardioid, or hypercard this way. Outdoors I do this with a spaced omni pair, and it allows me to space omnis twice as wide as I would do without the single directional mic in the center (6' instead of 3'). Alternately, or in combination, I'll use a pair of supercardioids spaced about 2' apart (similar to the photo above) but pointing directly to the sides 180 degrees apart. Remember that the presence of the microphone in the center requires the stereo pair to be more widely spaced and/or angled, than they would be alone. So 180 degrees apart may seem ridiculous, but if all three mics were cardioids that configuration would be identical to two DIN pairs sharing one microphone along their common edge. Inside this has worked very well for me from optimal locations, but from far back in the room you may not want to use omnis or supercards pointing directly at the side sidewalls. In that case you might try using cards or supers for the stereo pair arranged at a smaller angle and spaced more widely to compensate for the reduced angle between them. The logical extreme of that would be supers fairly wide-spaced but parallel to the shotgun, maximally excluding ambient room sound and focused on sound arriving from the front. You'll probably will want some angle between them however, even if it's somewhat minimal, like in the photo above.
Worth some experimentation I think, and you already have all the functioning mics you'll need to try it, you just need a recorder which can record 3 separate channels.