OK, I haven't used the EBC stuff but I know what kind of pad you're talking about.
The factory dampers are stick on pads about the same size but they have aluminum pieces that fit into the pistons on the caliper and hold the pads in place.
I'm not sure if you can use the two on top of each other, probably worth a quick phone call to EBC and see what they recommend.
If it were me, I'd probably pull off the flat shim of EBC's and use the factory style. That would keep the vibration down AND keeps that pads in firm place so they don't chatter when switching from forward to reverse braking. Maybe EBC found out that the piston inserts weren't necessary?