Your diagram with common ground is correct.
When I use separate power sources -- e.g. for retracts or swing-wings -- I achieve the same thing a different way: I splice in another black wire to the existing black wire on the servo that's going to get the external power supply, then one black wire and the white go to the receiver for the signal, and the other black wire and the red go to the separate power source. Makes it simple for me to follow, and a servo plug with two pins in it is less likely to fall out of the receiver socket than one with only one pin
If you don't wan't to mess with the servo wires themselves, you can make a separate harness that splits the wires the same way.