Can the DRD 455s be set to different IR addresses? If so, you could do that and learn the commands from each DRD 455 remote. If not, signals from the MX-700 will operate both if they're at the same location and the solution would be to get and MX-800 or 850 where you could use independently controlled IR emitters with the MRF RF base station.

Mike
www.SurfRemoteControl.com