Of course you can merge M/S channels to L/R files. You have plugins for this in WaveLab (look at the StereoTools). But in the “destructive way”, that is, to rebuild an audio file. The split you mention in the audio montage, is non destructive. Once you apply effects to the M/S channels, you can’t obviously merge them again “non-destructively”.
And Audacity won’t help you ther!
Thank you for clarifying that. Yes you´re right, its possible to do all kinds of splitting and merging both to LR and MS. Wavelab 9 is awesome in that sense.
This right click split is virtual. The original file is not modified. And the sound is merged back to stereo on the way out to the master section.
But, what I meant is, I wish there was a way to visually go back to the way the file looked before the split, and preserve the added modifications. But maybe that would qualify as a render operation.