You have the BEC connected to the wrong battery- it must only be connected to the main ground, and the positive wire coming from the same pack:
( ignore the rx and servo wiring as it doesn't apply to you )
This is how how Castle says to wire up the BEC when running a couple batteries in series. You could also wire it to the main ground, and main positive on the other pack so that both lipos power the BEC, but it isn't required as such ( a larger step-down in voltage results in the BEC running warmer and supplying less current, so running it from a single battery makes more sense ).