Need help with multi track/simultaneous recording

Please forgive me if the answer to my question has already been covered; I have searched the other posts but haven't been able to find the answer.

I am recording a podcast with a Behringer Xenyx X1832 mixer. In GB I know how to select "multi track recording" and set my two mics that are conneced to the first two channels of the mixer to "mono 1" and "mono 2" on the input source.

My question is how do I record audio from my Macbook on a seperate track?

I have a Focusrite 2i2 connected via USB to the Macbook acting as an output, then TRS cables from the outputs going to channel 7/8 on the mixer. When I play audio I can hear it perfect and adjust EQ and volume, but I am unable to record it on a seperate track on GB.

When I click on the track on GB, the input source says "stereo 1/2" and when I try to click the red buttons to arm the other two mono tracks where my mics are connected it says "The input channel of this track is already in use. To record on this track, choose another input channel."

What am I doing wrong? How do I send audio from the Macbook to the mixer and then record it on a seperate track, along with seperate tracks for each of the mics?