I did look at the HC-2. I didn't go that way primarily because
1) I don't expect to use the CB (though I might listen in sometimes)
2) I would need TWO of them for the passenger to be able to hear the same audio.
It's handy to 'dance' to the same song. 
The appeal of the SM10 which I did buy is twofold:
A). It broadcasts the same to each of two headsets
B). It will accept two inputs and it prioritizes them.
Using an AUX cable from the HD radio on Audio IN, I can hear everything the HD system provides including CB and XM.
My passenger also gets the same audio. If I plug any other device into the SM10's Aux IN, it will override the HD radio
when it sends output. Think Radar Detector, a GPS, whatever.
I have a Garmin Nuvi (not Zumo) that is a 'car' GPS (yes, they're different). This had no audio out so I added a switched stereo headphone jack on a pigtail as an experiment. I have this plugged in to the AUX on the SM10. It did operate in the garage; not yet road tested.
Here's what I've found so far:
Headset pairs with iPhone
Headset pairs with hub (SM10)
I can take a phone call as normal.
The headset mutes all other audio when on phone
When not on phone, headset plays whatever the hub is getting on Audio IN. XM, FM, whatever.
When my Nuvi makes an announcement, (on the hub's AUX IN) the hub lowers the volume of, but doesn't mute, the HD radio and sends the Nuvi's audio to the headset.
Really confusing trivial bit;you may wish to skip ahead. If I want to listen to the music stored on my iPhone, I can power off the hub and the iPhone will 'pick up' the pairing for the music. This loses my audio from my GPS, of course, because it is wired to the hub I turned off.
Alternatively (untested) I can use the audio cable that was plugged into the HD radio and just plug it into the iPhone's jack. That way I would hear my iPod music over the hub and still have the GPS.I need to sort out the volume settings to find the proper bias of source and amp.
If I crank the headset volume up and use the HD controls for volume, there's a buzz. If I try to control with the headset and leave the HD radio wide open, I get distortion.
A mid-level on both seems to work fine, however.
The next issue will be getting the GPS's output
louder through the hub. I believe there is a setting in the hub's configuration for this.
Keep in mind that this is all preliminary as I just got it on the bike yesterday. I am about to head out on a long-assed trip and will learn how well or poorly this idea works soon.