Double Data Rate Source Synchronous Constraint

set_false_path does NOT tell TimeQuest if you're doing same-edge or opposite-edge capture. All it does is tell TQ not to analyze the capture not being used. I tell users not to do the false path at all, because it only cuts timing on the paths that meet timing easily(in your case, you would want to cut timing on the opposite edge captures that make timing, which doesn't help that your failing timign.) The reason I recommend against it is that it doesn't help, but if you erroneously cut timing on the same edge capture, you'd make timing but your interface would fail.