Do not believe you can pair directly as you indicated. Which Sena do you have?

I connected my phone to the bike, NOT the headset. I then connected the bike "headset" bluetooth to the headset itself. Finally, I connected the Zumo to the headset as a GPS on the 2nd bluetooth channel. The Zumo will then interrupt the bike with turn by turn directions. Note I am assuming you have a Sena with 2 bluetooth channels, which is most of the older ones. I've heard some challenges with the new mesh intercom ones and available bluetooth channels.