The fastest option is to plug both disk drives into the same player and copy the FIDs folder as outlined in the Drive Upgrade Guide at www.riocar.org. You'll have to play a little game of Musical Drives to get the contents of both 20's onto the single 60, but it'll work.

Another option is to use Jemplode via ethernet to do the cloning. Faster than USB, or so I've heard. Dunno if anyone has successfully cloned 40 gigs in one sitting, though.