A mono synth with stereo outs is just 2 mono outs- a pseudo stereo situation. You do not program anything different for stereo, it is just how it gets amplified and distributed. Panning and stereo field modulation. The XS is monophonic as well, meaning it can only reproduce 1 voice at a time. Most mixers come with a group of mono 1/4 ins and a group of stereo 1/4 ins. Plus it looks like the XS has multiple outs, so you can route all kinds of things through it and out of it.
To simplify it, if you can change the stereo values of sounds from within your machine, it is stereo. If you have to effect it externally to modify the signal, that would be mono.
I do not mind questions, this is better than endlessly explaining how to get the witch castle vocals :P but maybe you should post a different thread since this is off tangent.
`michael