The SL 32SX is sending its Main L/R via US 39+40 to the DAW, so you need to create a stereo track in your DAW, route USB 39+40 to it as input and record your mix. At least this is the way it's done in Studio One. The song templates already supply a stereo track for Main L/R, but the routing needs to be adjusted. By default Main L/R in Studio One is assigned to USB 37+38.