You can’t connect them as a single mixer as far as I know.
You can make one mixer a submixer of the other and feed LR from it into a stereo pair on the master. But you would also need to dedicate channels on the main mixer for each separate monitor send you might need from the submixer I guess. It is not ideal but maybe you can sacrifice say 6 channels on the main mixer (LR + 4 sends say) and still have 26/28 channels to work with.
Or buy a QU 32?
Any other ideas anyone?