composite (RCA or A/V jack) output Passive splitter
Traditionally multiple devices would be connected in a daisy chain fashion with a terminating resistor at the end of the chain matching the cable, connectors and source impedance.

You will find that the typically 1.5V open circuit (0.75V terminated) signal will be reduced in value if you add two (or more) terminating resistors in parallel.

The wiring could be effected by the adapter you show and would often work if the device automatic gain control circuits are forgiving (a gamble).  You would have a much higher likely hood of it working if you were able to remove the matching resistance from the devices along the chain.  In older video equipment that had a passive line in and line out socket there would often be a switch that was used to connect the terminating resistance if it was the last device in the chain.  The terminator could also just be a resistor plugged into the OUT socket as was typical of Thinnet and Arcnet cabling.

Modern equipment is more likely to have a active (buffered) OUT connection that often may include injected on screen display information and this can be connected directly to the next device.

Alternately you could get a video fan out buffer that has a powered driver circuit for this purpose.