RC / Airsoft Battery Switch [On-Off-On]
Would you expect this wiring setup to work  


Yes.


  Am I correct in assuming that the Single Pole Double Throw switch would indeed need to be rated approx. 50amps?  


Indeed, that would be the right thing to do regarding selecting a mechanical switch. That would get very bulky and expensive.  

You might get away with e.g. a 10 A switch, as your application isn't exactly safety critical, you probably aren't holding the trigger down all the time or toggling the switch while firing and you probably don't need a switch life measured in hundreds of thousands of cycles.  

Airsoft guns generally use transistors to switch the motor current on and off, so that the trigger switch only has to turn the transistor on instead of some beefy motor. That's why the trigger assembly can be so compact. You could use such electronic means for your transfer switch, but I fear that it'd get too complicated for a layman. Diodes are normally very simple to use, but in this case you'd need some with a current rating over 50 A and they'd need heatsinking.

However, I'm skeptical about your need for a transfer switch in the first place. If you are just willing to extend battery life, you can just parallel the two battery packs directly. Note that the chemistry, state of charge and most importantly the cell count of the two batteries must match (You risk a nasty battery fire otherwise). This way the load is also shared among the two batteries.