Just put the mics where it sounds best in the room. The advantage of Double M/S is that it frees you of having to take other things into consideration when setting up.
It's helpful to not overthink the rear facing cardioid in a Dual-Mid/Side setup. If your target output is 2-channel stereo rather than multichannel surround, think of a DMS setup as "improved Mid/Side" in that it offers a choice, after the recording has been made,
of any X/Y setup (using cardioids, supercardioids, hypercardioids, or bidirectionals), Blumlein, or M/S (using an omni, subcard, cardioid, supercardioid, hypercardioid, or bidirectional Mid microphone), using any angle you want between microphones. In that sense it is super powerful and more of a "stick it in an appropriate spot without having to worry about choosing the most appropriate microphone setup at the time," because you dial those things in afterwards.
The most basic aspect to consider with DMS in regards to it's suitability and optimal recording position is that it's a coincident microphone technique.. and that's also represents it's primary limitation. It is capable of producing the identical output as any
coincident microphone arrangement, but if you generally prefer non-coincident near-spaced (or wider-spaced) arrangements it's not going to give you any of that.
I know that for running both MK8's in Blumlein I need to be as close as possible to the source.
Not quite that simple. Blumlein is tricky because it is completely omnidirectional with regards to directional pickup sensitivity yet has the most narrow stereo recording angle of any stereo microphone configuration with regards to the stereo image it captures of whatever is "in front". So a close recording position is helpful in the first sense, as a way of increasing the pickup of clean the direct sound without getting swamped in room 'verb, yet the narrow stereo pickup angle means that is can be difficult to fit all the sources within it's narrow stereo recording angle without moving further back. The SRA for crossed bidirectionality is less than +/- 45 degrees, compared to +/- 180 degrees for crossed cardioids setup with the same 90 degree angle between the mics.
But again, the advantage of DMS is that you don't need to worry about all that stuff when setting up. Just put the microphones where it sounds best in the room and dial in the virtual coincident microphone configuration which sounds best afterwards.
Here's a suggestion you might try a time or two which may prove interesting- Your DMS setup will consist of one CCM4 facing forward, one CCM4 facing backwards, and one MK8 pointing sideways. You'll need to record that to 3 separate channels*, so presumably you're using a recorder with or more 4 channels. Go ahead and use the 4th channel to record your other MK8, facing fore/aft and arranged coincidently with the DMS setup. Then you can compare native M/S Blulmlein (bi-directional Mid) with the virtual Blumlein derived from the DMS setup. You'll also get a good feel for how even a slight amount of adjustability in being able to tune in the most optimal microphone configuration afterwards can make a substantial difference in the quality of the resulting recording.
*Unless using the Schoeps DMS hardware decoder to derive a 2-channel output, but that eliminates the freedom to dial in the most optimal settings afterwards while listening in a controlled environment.
I just went to post and noticed DSatz had replied while I was typing. He likely covered much of what I just stated with greater expertise and insight, but I'll go ahead and post anyway before reading his post. I can always re-edit mine if I've stuck my foot in my mouth.