To test your packs, just turn on the first switch, check for proper operation, turn off the switch, turn on the second switch, check for proper operation, turn on both switches and go flying.

NOTE: If you don't want to carry the additional weight of a second battery pack, but would like some redundancy, just solder another wire onto your battery pack and run a single battery with two switch harnesses. Switch harnesses are much more likely to fail from vibration than batteries anyway....</p>