I am not familiar with the Freecom Headset,
but assuming it is capable of having 2 devices paired to it,
I would pair the Garmin Zumo XT to your Headset,
using “GPS Pairing” mode if your Headset has that feature,
and also pair your Headset to the Trophy “HEADSET 1”
Channel, using the Headset “Mobile Phone Pairing” mode -
if it has that feature, even though you are not going to
use a Mobile Phone in your setup.
The Headset “should” prioritise the GPS Audio so that any
Voice navigation instructions will interrupt any other audio
being listened to, such as the Trophy FM radio, then revert
to the other Source once the Announcement has ended.
Cheers

