In the new review Doug posted, he states,

"The TerraFix 406 GPS I/O (Interface/Onboard) incorporates an integral GPS receiver while still retaining the external GPS hook-up (protoype photos included here). That provides a potentially useful additional capability over having only an integral GPS"

I've never used a GPS (or PLB), so it's not clear to me why having an internal GPS in the PLB and an external GPS plugged into it would be "useful." Would the 2 units symnchronize their data or does it improve their ability to acquire the satalites?