A:AnswerIf I wanted to route the stereo signal from my interface into one monitor, I would probably use a 1/4" TS Y cable (dual male to single male).
Make sure it's a TS cable and not TRS because TRS would supply only the left channel audio to a single monitor.
This would combine the left and right channel audio into a mono signal, so the sound coming from the monitor would not be in stereo
