I currently have FFXI installed and want to install this private server without interfering with my current install of the game. How can I install another copy of FFXI and have them both be playable? I tried Sandboxie but it doesn't seem to be working.

All you need is the ashita launcher with the proper config settings and you can use the same install for the private server and retail, I regularly log in to retail whenever I see its a free log in weekend to talk to some old friends.

Yeah. Whenever retail updates, we do too so Ashita is not only your launcher, it's your aiming device for what account on which server you want to play with.