normally folks do one or the other. I can see too many complications trying to use both. Mechanical brakes should be plenty strong enough without any help from the motor for braking.

If you really must, then yes. Hook the brake servo to channel 3, and set the ESC to the brake amount you want. You shouldn't have to "split" anything. Channel three just runs straight to the brake servo. the ESC will be on channel 2 and the motor will get it's braking signal from the ESC. I imagine this would work for forward only, with no reverse.
J.