Overbridge and Ableton audio routing of L&R inputs

I have 2 of my synths connected to DT via L and R audio inputs. When I record something in Overbridge each channel gets recorded respectively to left and right ONLY. So I hear one synth thru one speaker and same is the case for the second synth. Can I have both synths as mono in the final recorded product? Or do I really need a mixer then? Up to now I used my DT as both audio interface and a mixer for my set up.

In Ableton when I add ‘Audio track’ it only appears as L/R single channel for both synths. Is there a way to mute or somehow separate them out?