From your description of your mixing it sounds like you have a handle on things and will move more quickly toward advanced multichannel techniques than most novice tapers do. Follow your ears.
As goodcooker suggests, its often simplest and least problematic to choose the better of two stereo recordings rather than mixing them together. Yet if it works and sounds good to you to mix them then there is no reason not to go for it!
The "problematic or not" issue basically boils down to mixing two recordings which weren't made in such a way that they were designed to combine well without problems. In that case it's pretty much a crap-shoot as to whether mixing them will make things better or worse. Unfortunately with tapers that's pretty much the general rule rather than the exception - making two stereo recordings each of which makes sense on its own in isolation, yet are sort of haphazardly setup with respect to each other, both attached to the same stand without much thought given to the spacing, angle, directional pattern and other relationships between the two setups other than having both of them point toward the stage.
There are things one can do with regards to microphones and their setup to help make mixing two sources made in close proximity work more successfully more often, if that's what you like doing. It helps to consider of all the mics you are going to mix together as essentially a single multichannel stereo array, which may or may not consist of two separate stereo recordings which will be combined.
Your description of how you ran the two stereo microphones in one hand-held mount sounds like you are using a mount which essentially places the two stereo mics in close coincidence with each other, and that's one way of avoiding weird mixing problems. Another is spacing the pairs or some of the microphones far enough apart from each other. Unfortunately, the more common and oftimes problematic combination is two seperate near-spaced stereo pairs, placed on a stand so as to be near-spaced to each other. Too many microphones that are neither coincident nor far enough apart from each other, all pointing in about the same direction and picking up the same content = a recipe for conflicts.
..it's made a L/C/R recording. I lowered the figure 8 (the side mic on the M/S shotgun) down a bit to keep relatively center. The stereo condenser is more wide sounding... much more distant and more crowd noise. The shotgun is far more focused on the band, more mono-sounding, and less crowd noise. Matrixed it has the detail of the shotgun and the openness of the condensers and keeps the band cutting through in the middle. Just my opinion but I like it.
*The thing I personally love about it is it sounds SO MUCH like it really did at the Pavilion. I really love the venue, and when I listen to it the ambience around the band, when I hear it play, I can visually see what I really saw being there. Real clean, the band sounds crisp, and it brings me back to that night when I listen. So maybe I'm selfish mixing this how I like it? but it is what it is!
You are already doing things which work in a way similar to what I describe with regards to microphone setup and mixing choices for a multichannel array. My suggestion in the other thread to use a shotgun in the center between a pair of spaced omnis is a real-world adaptation to the oddity of audience perspective recording. It produces a type of L/C/R type of stereo I think you would really like. The omnis provide the wide openness, you-are-there-ness, the ambience, while the shotgun provides the direct clarity, detail and focus. The two parts work together to help each other rather than fight with each other trying to provide the same thing.
Using a Mid/Side shotgun in the center makes this better by allowing adjustment of the Side channel level when mixing. That essentially does two things- blend the shotgun center more more seamlessly into the open ambient "bed" provided by the spaced omnis; and provide some tight Left/Right coincident imaging across the center of the stereo image. Typically, just as you describe, you'll use much less Side channel than you would if you were using that Mid/Side shotgun alone. That's because the omnis are providing most of the width and diffuse stereo interest. The Mid/Side shotgun then can focus on providing a touch of stereo-ization to the direct sound. Each pair providing something different enough that they aren't competing but rather help each other.
If you have a pair of cheap miniature omnis give it a try, spacing the omnis as far apart as you are able to. Shoot for 3-6 feet apart but that's not always doable so just do what you can.