This question is answered sufficiently, but it was revealed what I was trying to do... run multiple versions of iTunes on the same system non-concurrently, which is not possible because iTunes is not so much a stand alone app as it is integral with the system (see comments under my question at top). The reason I wanted to do this is because I am running 2 iOS devices at iOS5 with iTunes X, but I just acquired an iOS7 device, requiring iTunes XI, and knowing Apple's modus operandi of surreptitiously removing functionality, I did not want to blindly update my system's iTunes to 11.
Here's what I did instead:
- copied my entire iTunes Library to an external drive
- installed a near identical system on a 16GB SD Card, same username as my main system
- room on the card was tight, so I disabled SafeSleep mode thusly:
sudo pmset hibernatemode 0; sudo rm -rf /var/vm/sleepimage which gave me back a whopping 8GB of disk space
- downloaded the iTunes XI installer from Apple and installed it
softwareupdate and applied appropriate updates including iTunes patch
- made a softlink from ~/Music/iTunes to the duplicate iTunes Library on the external drive
- launched iTunes and signed in, and because of the near identical system, Apple didn't even recognize it as different, so I did not have to authorize a new system
Now I can sync my iOS7 device with iTunes XI without affecting my main system and the iTunes X install there by booting from the SD Card when I wish to sync my iOS7 device.
I realize that eventually the 2 iTunes Libraries will diverge enough to become annoying, but it should take awhile as the only thing that changes these days are the apps I download for iOS (my music library doesn't change that often).
I'll have to manually keep track of what changes I make within certain apps such as GoodReader and iBooks to keep them in sync on all the devices (or I could use iCloud to backup app data and keep my apps synced that way), but at least I have local storage of backups of my iOS7 device, and can sync my music, books, movies, etc.
The situation is not ideal, but I am extremely pleased with it as I won't be backing up the iOS7 device on a very regular basis, and I find iTunes XI to be deplorable (iTunes X is bad enough for its bloat and feature creep, its slow, but it lets me do what I want... but iTunes XI removes nearly all user control, has inscrutable automation routines that can't be stopped, and is overall what I would call "junk software." I'm not sure how other users tolerate it! Doesn't anyone miss being able to have the autonomy to be able to control how your iOS devices are managed without having to fight the software for that control? What planet am I on?!?! :P
I appreciate the help that the responders and commenters to this question gave, as it was necessary to know what couldn't be done before I could do what I could do.