To select a specific disc by number, you have to go through the shitty steps of 'Display', 'Right Arrow', 'Key Disc #', 'Enter', 'Display', wait 2-3 seconds and 'Display' again. You may be able to skip the 3rd 'Display' if you are not displaying Bitrate or Layer Info.
One good alternative is to place the first disc in Slot #X and the second disc in Slot # X+1 and just use 'Disc+'. If you don't have the IR codes, download my CCF file which you can import into PENG or PPENG. This CCF file (Pronto Files Section) has all function codes for all command modes of the 777ES and 985V as well as other non-jukebox changers (non-jukebox or older DVD Players use command mode 1).
Sony does have discrete IR codes for Discs 1-5 (to support older 5-Disc changers). You can find these in my CCF as well.
[Link: remotecentral.com]