hugland,
Good practice to carry a backup in the woods and the small form factor/weight of the GTM makes that easy.
However, even with a single GTM that’s already paired for your own use, it will still act to relay messages so long as it isn’t sending or receiving at the same time.
But placing your second GTM up top of your gear powered on to act as a relay is still helpful and with the extra hops now available, more powerful than ever. If you carry a rope or even better the makings of what I call a Firefly then you’re set for even wider reach if needed. Having that second GTM available set to relay effectively acts as a ready to use antenna, no coax necessary.
While such a rig won’t relay Shouts, so you’d need to know the GUIDs of others to link to them for ordinary priority messages, Emergency Shouts do relay. If that’s why you need it in a emergency, then go for it as if you are still able place it up high it could make a substantial difference in your range.