Control Room with UR44?

I have a UR44 and use Cubase 9.5.21
I have added 2 mp3 reference tracks to my project and I want to be able to switch quickly between either of them or back to the main project
The Studio is set up with a “Stereo Out” channel routed to “UR44 Mix 1 L/R”
I routed the reference tracks to a new “Stereo Out 2” channel thinking this would allow me to route that to the "UR44 Mix 2 L/R"on the UR44 but I cannot hear anything.
I have a single set of PreSonus monitors on the back of the UR44.
So my questions are: What exactly is the “UR44 Mix 2 L/R” output that is available on the UR44? (I’m confused)

Line outputs 3 & 4, at a guess?

I accomplish what I think you’re after using 2 Stereo sub-groups (which then feed the Main Stereo out). The reference track goes to one and all the groups/tracks of your work goes to the other. Easy to switch between the two then.

look at this
https://www.youtube.com/watch?v=VH1tWdTddE4

Edit: basically with your setup,when you create stereo output2 in cubase, route the same output of UR44 that connected to your monitors to “Stereo out 2”

Thanks both for your help. I think I have it sorted now. The link gave me the answer, thanks @mozizo