I have followed the manual a few times with no luck. I plugged in the stereo cable from the 651 to the 652 and cut both on. Hit program then glob, clone, select. The 651 says filesys clone. Waiting start but never starts.
The above link shows the steps for cloning using a stereo cable. Make sure you're using the PC/IF jack and not the earphone jack. I've done this several times and these instruction do work.
Or you could just do as @lou9155 suggests, and just load a saved programming file from one scanner to the other. That also works well.