Just a slight diffrence in voltage between the 2 batteries is probably enough to make it work.
There is a farm shop near where I used to live that has a old phone in the farmshop (honesty box type shop) and it worked for a long time off a car battery, the 'cables' where the top and middle wire of a fence and ran back over 1/2 a mile to the farmhouse, worked a treat, I managed to get the phones to ring or should I say tinkle using the hook switches.