A good HDMI design has a chip right at the output that, among other things, keeps two sources from shorting each other out. However not everyone uses these chips.

If everything goes well, the two sources will try to determine what type of display the other is, but neither will respond to that query since they aren’t “sinks”. Then they will refuse to send video or audio, instead acting like there is nothing connected.

If one or both are incorrectly implemented you could burn up important circuits, or if you’re luckier, the cable could burn.