M/S in the center + split omnis to either side has worked well on stage for me a bunch of times. It's a quintessential 4-channel OMT setup as far as I'm concerned, working well both on stage and from the audience for me.
Somewhere I have a Charlie Hunter recording I made way back when using the TetraMic in the center between 4 split omnis. It was the very first time I ran the TetraMic, so I retained the four omnis I had already planned on using. I recorded them to separate recorders and remember playing around with both to compare, but never synced them up and only used the 4 omnis in the mixdown I provided to a video taper, then never went back to revisit possible combination of the two. I should go back and dig that up.
Two split M/S pairs is an interesting approach I've not tried. I have read of its use by someone for live classical recording - reportedly being particularly useful when there is a center isle that cannot be blocked with a centrally placed stand. That's probably when I'd consider it instead of having something in the center. As I recall, the mixing was described as panning the outermost channels from each pair hard Left/Right, with the inside channels from each pair being summed and panned center. A split M/S pair using omni Mids is sort of a logical extension of a pair of split omnis alone, and should provide additional flexibility afterward. I think how well it might work may hinge on how well the inner-facing channels sum, which is sort of a mono-compatibility question. Might have to just give it a try and see. If you are able to get a soundboard feed you will be better protected from potential "hole in the middle" if the summing of the inner-facing channels doesn't work out well.
I've only run twin M/S pairs once, but they were front and rear-facing MS pairs between spaced omnis, done on a whim one day at a fest by adding coincident 8's to the center front/rear facing directionals I already had in place. I don't think I ever got around to listening to the rear facing pair, recorded to a separate recorder. I should dig that up too. I went on to modify my regular setup by retaining the front facing M/S pair, yet went with a near-spaced rear-pacing pair. However, I've recently been thinking about that again as a way of streamlining what I have aloft while retaining stereo rear-facing pickup of ambience/audience/room.
