Is it ok to use two batteries in the same blackberry. I have 2 so when one runs out I can pop the other in and vice versa. Verizon told me it saves the life of the battery if you let the it completely drain before recharging it. Let me know if I'm hurting anything by using two.
There is no problem doing this but I suggest that you swap the batteries regularly, say once a week. Lithium-Ion batteries do not have a memory effect but the do have a shelf life which means that if you have two new batteries today then the probability of both your batteries failing at about the same time - within 3 months of each other is probable but there is no guarantee.
I do this with three batteries because I bought a 2 pack on ebay for not much money.
With Ni-Cads and Nickel Metal Hydride batteries you were supposed to run them down but I heard with Lithium-Ion you should try to recharge at 20-30 percent.