Use multiple commands in a single keyboard shortcut
Solution 1:
Only a single command indeed can be entered into the shortcut definition dialog. Two workarounds are 1) call a script that contains the commands and 2) use the construct sh -c "...".
In your case with only two commands, option 2 is simplest:
sh -c "xrandr --output HDMI-0 --off; pacmd set-default-sink 1"