As an update:
Fully working. For whatever reason turning on the newer TiVo as a server device in the Pics/Music section never causes actual directories to appear - but it does cause malformed discovery packets to trip off and the older TiVo can magically see the newer device. Got me hangin!
Plugging in the Static IP of the newer TiVo was a suggestion from a senior advisor over at the TiVo forums and lacking any other clues, I tried it and bam - it worked.
So give that a try if you hit a wall having one Tivo try to 'see' others on your network - make sure all devices are done with Static IP's and not DCHP as that seems to have some issues on any device by phones. Then go to Music, Photos & Showcases> Add Server and punch in the IP of the device(s) you can't 'see' on your (more than likely) older TiVo. Wait for it... wait for it... maybe even reboot them/your network. It's magic! Start looking for malformed packet errors in your logs, if you see those and it doesn't show then that's a whole new question.