If I recall correctly StephenH rides brevets a on his tandem so I anticipate that battery life is an issue. Any GPS computer has a relatively short battery life before requiring recharging. There are non-gps computers that read the same ANT sensors, will go thousands of miles between charges, and use replaceable batteries. They are much cheaper than GPS models as well.
I have tried a Bontrager model and had constant trouble trying to get it to read the sensors. I recall another poster having the same problem. Bontrager support was nonexistent. I suggest avoiding any Bontrager unit.
My stoker has always used Polar from back when she was running. She found one with a really large display which is easy for her middle aged eyes to read. It works well and the battery lasts about 5,000 miles but Polar does not use ANT protocol so we have to have two sets of sensors on our tandem.
I just purchased a O-Sync Navi2coach GPS with a replaceable battery. It is basically similar in function to a Garmin 500 except you can turn off the GPS. With the GPS on the battery life is similar to a Garmin. I have just put about 10 hours on it with the GPS off and the battery indicator still shows all bars. I think it will las longer than my wife's polar. Cost is about $250 since it has a GPS. It uses a standard cell phone battery. I purchased a new one as a spare from Amazon for $5. A stand alone battery charger was $15. Here is a review of the unit.
http://www.dcrainmaker.com/2013/05/n...-computer.html
I would try a non-GPS O-Sync like the Navi-move. I have not tried it but I love my navi2coach and am glad I did not buy another Garmin.
There may be other non-GPS ANT compatible computers on the market. That is the direction that I suggest. Longer battery life and much cheaper while using the standards ANT sensors.
I don't like purchasing equipment with a battery that the user cannot replace. It seems to me the unit is designed to have a life limited to the life of the battery. I think that is about 500 charges if treated properly. Perfect designed obsolescence.