Yes, you could run 2 omnis and a card into an MX-100 and get a great result. CapnHook has a recent NRPS show on the LMA which runs like that and it impresses the hell out of me. But, it's not technically M/S.
Mixer issues aside, there is a fundamental difference between the figure 8 sideways and the healy omnis... the math and physics are completely different. I'm not saying it's wrong, or bad, just that it's not really figure 8, therefore it's not really M/S.
The L and R side of the figure 8 are in one capsule, and one wave form, which grabs from the sides and very little from dead ahead. In post you break this up into two seperate wave forms, identical except one is upside down. The healy omnis are for the most part in phase, and will pick up from dead ahead.
Here is a sales pitch, but it has a good picture.
http://tracktion.mackie.com/products/800r/midsize.htmlI recently did a "real Mid-side recording" at Garaj Mahal, with the mic stand just on the stage, hence I'm pretty much micing the band's instruments, inside of the stacks. I recording the raw Mid > Left, and Side > Right, and mixed them at home into Left = 40% mid + 60%side, Right= 40%mid + 60% inverted sided. I liked it.
I also recently taped Little Feat from the audience and ran two similar rigs... 2 sets of Nak CM-300s, one with cards NOS, and one with omnis healy. Then I brought them home and listened to them. The healy method had some appeal to it, in particular a little more bass and ambience, but I ended up seeding the tape made with the cards, mostly because it had less audience chatter.