Theoretically, you can put almost 4 hours on an SACD only disk - but I've never seen it done. The dual-layer disk time limit is going to be restricted by the CD layer which is supposedly 80 minutes, so at 79:41 that's really getting close. One reason for using two disks is that some older players just cannot handle that long of a cd. I had a Panasonic years ago that burped when disk length was over 75 minutes. Nowadays, there are disks that exceed 80 minutes, going to 81, 82 and such. But it's risky. If everyone had state-of-the-art players it wouldn't be an issue. Another reason is that two disks allows them to charge more, if not double, to help defray the costs of recording.
I'm glad to see the Utah cycle appearing, and especially on RR - they generally have spectacular sound, although their older HDCD format to my ears sounds a lot better than even SACD. But try getting an HDCD player these days! It's also kind of sad that apparently we won't be getting a complete cycle from Pittsburgh. Every one of the recordings Honeck made there so far has been superb.