For what it's worth we sometimes carry 3 GPS devices on our bike, it is a tandem.
1) An old Garmin eTrex when we are following a new pre-planned route, this might supplement a paper queue sheet if we are out of our area.
2) A deactivated Andriod phone (WiFi only) that serves as a flight recorder, mp3 player and handy camera.
3) My active phone in a handlebar bag, used as a phone, but on occasion I'll use Google Maps to figure out where we are or if we want to reroute.
In fact I have 3 or 4 deactivated Andriod phones with ridewithgps that we use as flight recorders for my stoker, we usually get 2-3 rides out of a charge. Ridewithgps saves the ride tracks until you get within WiFi range then uploads them.