Are you doing this? - Running your L1® Compacts in stereo?
If you are running your L1 Compacts in stereo, then the connections are simple.
Connect the Mackie Left output to one L1 Compact.
Connect the Mackie Right output to the other L1 Compact.
Connect the headphone out from Mac headphone output jack with a cable like this to a stereo input channel 5/6 or 7/8