“I guess at a more basic level I was thinking about the digital bitrate and level of compression applied to the data on HD1, and how using more bits for HD2 presumably has a negative impact on the quality of HD1.”
I believe that that argument is *also* valid. There *are* several modes for hybrid transmission, Indeed, in the various “extended hybrid” modes, more digital sub-carriers are added to increase the permissible bitrate that can be delivered, with increasing anomalies being the result, as AB described.
It is also the case that within a given mode, the broadcaster can choose the bitrate to be applied to each stream… use the whole standard mode 96k for one really nice sounding stream or split it 48/48 or 32/32/32 or other combinations for multiple streams, with decreasing sound quality per stream.
Nice Ibiquity “White Paper” here detailing the FM sub-carriers in various modes: