The Behringer XR12 should be good to use with two S1 Pro speakers - simply connect from the mixer's stereo XLR outputs to the S1s.
For best practice, make sure that no EQ or ToneMatch presets are used on the S1 so that any processing is getting applied at the mixing stage on the XR12.
Hope this helps!
Feel more. Do more. Be more. Come check out my profile here. New to the community? Check out the Getting Started board. Help others find answers to your questions by clicking on 'Accept as Solution' at the bottom of a reply. If you see a post you like, make sure to give it a thumbs up!