Hello vincentrapide. In follow up to my original post, I have gone ahead and purchased two Intaride S-3 headsets and a new TomTom Rider 550. Having been assured by the owner of Intaride that my desired setup would work, I took the plunge, plus there is the security of a 14 day returns policy!
I have tested the setup and can report that my pillion can hear the TomTom instructions and we can talk to each other without the need for any BT switching or pairing complications. So long as you pair the TomTom to the rider first and then pair the pillion to the rider's headset, all communications are audible by all parties. I have not paired anything to my iPhone, so cannot comment on whether music can be heard, but I would have thought that it would be.
Obviously, I have no experience with any other type of headset, but the pairing was relatively easy and the fact that my pillion can hear the TomTom is great.
However, the downside is that my pillion has reported that the quality of the audio/ voice is somewhat poorer than if we weren't paired with the TomTom. This is odd as my experience of the audible quality is pretty much the same whether we have the TomTom paired or not. She can hear stuff, it's just less clear.
Also, we have noticed an annoying time lag between the TomTom and the headsets. Bluetooth lag is a thing apparently. Turn instructions arrive to both of us a bit late, sometimes up to 50metres later than the displayed distance to the turn is indicated. We have learned to compensate!
Hope this helps.
Chris