In principle, yes, assuming 6-channel + 2-channel equates to 4x 2-channel, you could put some 280 minutes of stereo on a single layer SACD, and since dual layer SACDs are possible you would end up with some 560 minutes of stereo on a single disc. Unfortunately, other considerations were taken into account when the SACD specification was created, and so the maximum length of a disc is limited to 256 minutes.
Hope that answers your question, Demery (Philips Electronics)