Could also use a 'flying cap' circuit if you only want to measure the 'other' battery periodically.
A capacitor (say 100uF), connected to the two common contacts of a DPDT relay, is connected at idle to the battery with the two NC contacts, and periodically switched to the Arduino (the NC contacts).
Switching, measuring, returning, would only take ~25ms when a tiny DPDT relay is used.
Leo..