UR44C - How to use 2 output channels?

Hey guys, I use my Steinberg UR44C as a soundcard and I also connected my guitar to input 1 (but I only use the guitar in DAW mode, so I turned down all input levers in the DSPMix-Software).

I also connected two pairs of speakers, one to “main output”, the other to " Line Output 2"

However, I only can hear the speakers that are connected to “main output”. How can I direct the DAW sound to come from both pair of speakers?