Common the negitive wires of the RX's. Power the new gimbal rx and servos with a new ubec.
I common the rx negitives as... I like to control my gimbal tilt solo from my Tx and (at the flick of a switch) give control to the camera operators Tx. The pilots rx tilt channel (signal wire) goes to one side of the switch. The Cam ops Rx tilt signal wire goes to the other side of the switch. The common (signal) from the switch goes to the tilt servo, and picks up the +ve and -ve from the gimbal ubec on the way.
Lot of wiring of single wires but very effective. I can solo control tilt and shutter and swap to the camera op's RX with a single DPDT switch

Always power the camera servos with a different ubec from the FC rx.
Pete
p.s. Sketch it out on paper first and lable (dymo tape) as you build. Fault finding is so much easier that way
