I read that using the pixhawk the pin rail to power devices like servos is not a good idea as it may cause power problems. Is that true?
Yes indeed! Beside why would you like to introduce possible electric noise from servos to a flight controller?
Look at real aviation: They never mix power servo and computer power… They have exactly the same reasoning. 
So for best results separate power from servos and receiver/flight controller.
(The worst possible: Powering receiver and flight controller from an ESC!!!)
Cheers
Henri
I have used an existing power source and connected the two digital servos. but whenevr i trigger them together at the same time a lose momentarily video signal and my osd telemetry disappears. is this because the 2 servos are triggered at the same time?