Assuming you are using two lithium batteries at 3 volts each, removing one will decrease your voltage by half, therefore also decreasing your current consumption by half. So in essence if you used one battery at three volts, it will last twice as long on the same bulb, than two batteries at 6 volts. Hence you will get 4 times as much life total out of two batteries used individually (3v), than using them together (6v).
I understand that this still might not be the ideal scenario for you, but saying that it doesn't extend battery life, is simply not true.
