For SACD, all the multi-channel SACD's I own, also include stereo 2 channel tracks. No downmixing of the MCH version is necessary. Since I don't have any DVD-A discs, I can't say whether or not they include 2 channel mix as well.
However, if you're talking about using the Multichannel output but without the center, then I can't help you there. Since there are not a lot of chips that do DSP in DSD, have a phantom center may not be an option in most SACD players.