I don't have a Cardo, but I think the principle is the same.
You have two BT channels on your BT headsets and two on your Garmin XT. How you chose to pair those two, is up to you. Chose wisely as someone more famous than I am once said.
I leave my Garmin XT on the bike all the time while riding. It's literally a "snap" to put it on and take it off.
- Primary BT channel is connected to the smartphone.
- Secondary BT channel is connected to the BT headset.
There is no direct BT connection from the BT headset to my smartphone. All phone calls route through the GPS.
My BT headset has two channels.
- Primary is connected to the radar detector.
- Secondary is connected to the GPS.
If I'm using "Group Intercom", then the primary BT channel seems to go to the Group Intercom and not to the RD. Remember, you only have two BT channels, no matter how much you wish you could get 3 or 4.
Virtually all my music is stored on the Garmin XT. I don't try to access my music on my smartphone. Music, phone calls, etc. are all run through the visual interface on the Garmin XT.
(I have to give a little caveat on all the above. I'm on drugs for something I'm allergic to and hives. Who knows what reality is, and isn't.
)
Your Cardo app (I'm assuming) is assuming you have no other device you're connecting to. They want you to be tethered to their app and theirs only. And it would be impossible to program for every other manufacturer's options. Skip the app, if at all possible.
I used to connect my smartphone to both the BT headset and the GPS. The results were random. It all depended on which one paired first as to what the final outcome would be.
I hope this helps.
Chris