Hi. Sena 10s user here. I tried for a long time to pair the 10s unity with a friend's Cardo unit. Here's what we finally did. Basically, this process makes the Cardo think the 10s is another phone.
On the Cardo:
Put the unit in "2nd mobile phone" mode
Hold phone button until it blinks blue/red
Tap phone button twice and it will blink green/red
On the Sena:
Note: Disconnect your phone and turn it off.
Put the 10s in Universal pairing mode: Hold the jog dial button down for a long time, maybe 12 seconds or so, until you hear "configuration menu." Use the jog dial to scroll to "universal pairing mode". Press the phone button (center button on jog dial); 10s will start blinking rapid blue.
Units should pair after a few seconds.
After units are paired, you can re-connect your phone to the 10s.
Units can be turned off or disconnected by distance and will re-connect automatically from then on.
This is what worked for me. If anyone else has a better explanation or simpler process, I'd like to see it.