Today I managed to get the two headsets to pair with each other by a) with each headset switched on, holding down the Ctrl button for about 10 seconds until there was a red/blue flashing light sequence, and b) holding the 'volume up' button for about 5 seconds to delete all pairings. Then I managed to pair the headsets by switching on and holding down both buttons together, until I got the same red/blue flashing with both headsets, which then recognised each other and returned immediately to a slow blue flash. I then tested that we could communicate, and after success, held down the Control button on the main headset until it flashed blue and red, then went through the menus on the TomTom Rider to connect a new headset (motorcycle). I can't believe it finally worked, and I can now hear instructions over the headset. Guess I'll have to wait until I get a new phone for the phone to be available though -the E51 isn't supported, for some inexplicable reason.
PostScript: I had problems with turning on comms manually, so (on advice from Cardo) I reset the pairings by holding down control (with unit turned on) until it flashes red/blue (flashes red first, but persist, keep holding!!), then holding 'Up' audio for 5 seconds. Paired with the other Q2, but then couldn't pair with the TomTom. After about 10 'required features not found' messages, I deleted all pairings again and tried to pair first with the TomTom. Success. I'm now going to pair with the other Q2 again, as per this post at PocketGpsWorld. Halfway down there's an entry by 'Ariel Lifschitz' from CardoSystems support. He states that the order of the pairings is crucial.
By the way, it didn't fix the comms initiation problem. I still have to yell into the mike to get the comms started. Used to be able to just press 'control' and comms would stay open until I pressed it again. I'm comin' at ya again, Cardo!!!!!! AAAARRRGGGHHH!!!!
Christ, life's hard sometimes ;)