The LP-16 does not have an option to mix audio channels to independent output. Each track assigned to one output.
But with some trick you can solve that.
In uTool when you create the song you have to set up the levels for your downmix, and mute the tracks (clicktrack) what you don’t want to listen in the downmix.
Use the Headphones output to listen your downmix and use the channel output for the clicktrack