I'm planning on getting EagleTac G25C2 Mark II with its kit. It includes an extender option to use 2 batteries instead of just one. I assume the only benefit is longer running time as the capacities get combined in a certain way. So for this, I thought of getting a pair of KeepPower 18650 3400mAh batteries. My question is, once I have these, should I always use them together in the light, then charge both at the same time? Or is it okay if sometimes I just use one of them (causing different level / more wear on one, not that much on the other). Alternatively, should I buy 3 batteries so 2 of them is a pair and the single one I'd use by its own, when I need a compact light?