I would put the synthetic layer below the down. The syn will compress less than the down, so the layering should be more efficient. If you are layering something like the pine quilt with sewn through baffles, offsetting the baffles would be best. I just bought a KAM snap set so I can add some snaps to union my layers. I have a Nemo tango duo that is good to the 40's. My plan is to put the pine down under the tango with the 82" going width wise, attached with the snaps to hold in place, so we can tuck more around us. Going to use some very light webbing and sew that to the quilts, then add the snaps to the webbing.