I don't say when you have 2 EN-EL3 in the grip, it only uses power from one battery. But I do like to know whether they use it both in the same time, or use it one by one.
It just if you only have two batteries, you'll get both of them exhausted in the same time, as you've said. Meaning, after that you have to wait for battery charging. So the scenario is I have to have at least 3 EN-EL3. When the first two is empty, you still have the third to do your shooting. But in my case, I have only 2 EN-EL3 I get to insert one battery only.
Pinky Mirror Nikonians@Jakarta,Indonesia-- it's near Bali, if you don't know where it is