I have odd question for you more experienced folks out there. Would it be possible to take a ready to fly drone such as the Phantom 4 or Yuneec Typhoon H and attach an extra flight controller and receiver to the battery that is already on board and use them to control attachments such as servos or LEDs with a separate transmitter (given that it is on a different frequency than what is already on board)?
I would say at least with the Phantom 4, no. I had one of those and they are crash and flyaway-prone on the best days. If you’re a Phantom pilot you’d better keep your finger on the Atti button to take control of it at any given instant or you’ll eventually lose it. Adding anything to one of those will likely make it crash.
Don’t know about the Typhoon H - have no experience with that one.