I think that you just want to assign a pair of the DM's outputs to Stereo-L and Stereo-R. You do that from the Output Slot on the DM's Routing page. I would probably use a pair of slot outputs that I don't normally use very much, like 25&26. Make sure that you have a stereo track in Logic assigned to that pair of outputs(input 25&26). Just be careful with your levels, you will have two copies of the same input going into the mixer, so there will be a possibility of feedback if you haven't isolated the two stereo signals.