You will get more than what one would put oiut, but it won't double. You'll get somewhere around 75-80% more. The reason for this is due to how the feedback works in the switching regulator.
Yeah, I personally wouldn't parallel BECs at all. It depends on the design of the BEC circuit really. I've had a BEC (can't remember which) get really hot if the output was fed with even a slightly higher voltage.
If you have a high servo load, I would just run a seperate BEC on just that device. It's just a matter of isolating the red wire, but the ground and signal wires need to be directly paralleled.