Originally Posted by 6thElement

Ideally you need both shifters functioning with SRAM as they individually move in either direction with the rear derailleur.

The right shifter on my wife's Force D1 was bad and would flatten the battery if we left it in there. We both found it easy enough using a penny to install/remove the battery for every ride. I eventually picked up a replacement shifter and the old one sits in spares in case of emergency.