china_rider, no, I don't actually have anything very wonderful worked out for this. For that matter my computer and my stereo system are in two different rooms and aren't connected in any way, so it gets awkward whenever I have to do any critical listening to stuff that's on the computer--I end up burning test CDs and running into the other room to play them. Also, while I described some tests that could be run to check out one's M/S software, I've never actually done them myself (yet). I'll get around to it some day.
I happen to use Sound Forge for most two-track editing simply because I know it best. I think Adobe Audition is probably a better program in most respects technically, and it has a nice M/S arrangement while Sound Forge has nothing built in for that. But it takes me a long time to really get to know a complex piece of software, and I've only had Audition for about two years, while I've been using various versions of Sound Forge for maybe eight or ten years now.
M/S is a nifty technique, but it's not actually my favorite way to record most music. It has essentially perfect mono/stereo compatibility plus the obvious ability to let you make certain decisions about image width and reverberance "after the fact"--but so do any other coincident setups (since you can always matrix any form of X/Y into sum and difference signals which in effect are M and S). For most non-commercial recordists mono compatibility simply isn't an issue, since they always listen in stereo anyway. It makes much more difference if you're doing film or video sound, or recording for radio (which I used to do quite a lot of).
With M/S recording I've always found that there's a pretty narrow range of M:S gain settings that give a plausible result. It's usually like--I turn the gain knob and find the center of that range, and I'm done; the other possible settings just don't seem all that useful. Double M/S shouldn't have that limitation nearly as much, but I haven't had a chance to experiment with it yet.
--best regards