Going to your initial question of benefit between two 600maH packs or one 1200maH pack, you will see no benefit at all. The percent capacity draw from the two packs will be the same as just the one.

Now, with all of this said, you can have a redundant power source by hooking the two packs into the same receiver through two switches. If you are using cheap switches, you may see a lower voltage drop, but with a plane that is using a 1200maH pack, you should not see any great voltage losses.

Last, if you are using digitals or a lot of servos so that you could exceed 2 amps during flight, then you may see some drop do to your electrical connections.