Read encoder output by using an optocoupler
People sometimes use optical couplers when anticipating large voltage differences or the potential for large voltage differences.  This may always be the case as in a light dimmer.  Or may preventative as in ground loop mitigation for MIDI connections.  If you anticipate these types of situations then an optical coupler may be a good design choice.
Alternatively if the signal is changing between 0 volts and 5 volts and the fan out of the signal is larger then the existing load and the new load combined and the new load is 5 volt tolerant then the signal might be able to drive both loads.
You might seek out the specifications of the 3 connected devices in question.  At the very least, discover the current the encoder output will support.  Then measure the resistance to ground of the input to the motor controller and the resistance to ground of the input to the video equipment.  Find the parallel resistance of these two resistance and calculate the current at 5 volts.  If the current is substantially less then the specified capacity of the encoder, the encoder might be able to drive the 2 loads.  This assumes many things such as no ground loops being present.  There are so many variable it is difficult to be predict the outcome.