Batteries are relatively inexpensive and using them from the get go won't do any harm. Charge one battery with the PCC plugged into the usb/plug. Start vaping on the other one.
At some point, your eCig will start blinking letting you know the batter is discharged and even if you try to take a couple more hits/drags, it won't really let you or you won't get anything out of it. By then, your other battery is ready to go, keep the PCC plugged into the usb/plug and just hold down the button for 2 seconds and it will start charging the battery again and once that is done, your PCC will continually charge.
As Phoenix stated, there is no memory in the batteries.
There is no technical specification behind my statement here but by the time your battery to truly die completely, it would be time to purchase another one anyway which would be several months from now.