Yes the stereo S/PDIF transfer not only occurs over one cable, but occurs in real time. So an input channel receiving this stereo signal will get the digital version at the 1x speed that is inherent in analog transfers. The designers were obviously looking to make the digital connection almost transparent to analog users.
I'm thinking that the protocol for this standard is to send two (or 3 for 24 bit) bytes of a left channel sample followed by a matching group of bytes for the same time slot in the right channel, then repeat until the file has been transferred.
From what I've read in the TASCAM CDRW-5000 manual, track information is indicated by a stream of zero samples for a specified time-out period.
Just a guess based on the properties I've observed while using this format.
Since computers can transfer data at MUCH higher rates, it's clear that the standard was premature. But it still works great!