MátéNovák wroteThanks, I checked the UC but I can only set the sampling rate from there. I can set stereo monitoring from Studio One but the interface handles the inputs as 2 separate mono tracks by default.
So I guess I need a mixer or an additional pair of speakers in this case.

To get to the mixer in UC, just left-click anywhere in the window that's not a drop-down menu, like on the image of the interface. The mixer looks like this (for my 1810c; yours might look a little different). Click on input 1, and then click on the stereo button top left (looks like an infinity symbol). It will toggle between Ch. 1 showing panning in the center or Ch. 1 and Ch. 2 with Ch 1. panned to the left and Ch. 2 panned to the right. Note that in the first image, Ch 2 disappears. You'll have to try both settings to see which works for you. Hopefully, one of the two will.