I have a Leightronix Pro16 controller for tape broadcasts. It sends
play-rec-rew-stop commands to 8 tape decks, and switches the video and stereo
audio to any of 3 outputs.

Since I secondarily use it as a dubbing switcher (output 2), I would like to
keep the audio in stereo. But I only feed my cable RF modulator in mono. The
manufacturer warns NOT to use a y-adaptor to do this. Is there any other way to
mono the stereo output (1) without any adverse effect to the other outputs?
Would a small mixer do it without any "backsplash"?