If the quote is true, then you always have the servo as a load on the battery, even when the switch is "OFF". Not a good idea.
As to a comment in another reply above, two batteries in parallel DO NOT self discharge any faster than either one will alone. However, the resulting impedance of the power system falls (a good thing).
Most of the above suggestions work, but if you have no way of checking before each flight that both systems are working (with one battery switched off, you should have full control. Now turn the other battery on and the first off, you should still have full control. If both tests are okay, they turn both systems on and fly. However you set them up, make sure you never try to charge the two batteries in parallel. Nixx batteries will not properly charge in parallel without some damage occurring in one of the batteries.