Can you use this with single M/S?
Nop. The whole idea is how you mix the forward and backward pointing cards. Adds nothing if you only have one mid mic.
Theoretically (always remember that theory and practice are two different things).
1) you can describe a cardioid as having one figure 8 and one omni mic (some early ones actually were made this way).
2) if you have two of these cardiods, back-to-back and add equal amounts from them, the 8 part is removed and all you have is the omni.
3) if instead you subtract the two signals, the omni parts are removed and all you have is the figure 8 part.
So in effect, by simple means you can modify the M mic to go from omni over cardioid to figure 8 and all the patterns between. This is what underlies the independant adjustment of stereo width and pattern.
// Gunnar