Thanks, using a Surround track is a very interesting idea. We don’t have a Surround decoder available, unfortunately, or I’d give it a try.
I have been able to achieve what I’m after using Ardour and Jack. I got the idea here:
https://forum.audacityteam.org/t/how-to-for-multi-channel-out/1252/1
I assigned each click track to a separate channel of an aggregate device of Built-In Output and my two-channel interface. I understand, as you say, that these two devices will tend to drift out of sync. I’m hoping that since all I need is a headphone feed from each device, that any drift will not be consequential.
Thanks,
Dave