Sorry missed that bit.....
Not sure about 'never' . . . . How big is your library?
My instinct would be to go for a 2tb drive. If that's not big enough then a multiple bay RAID external or (albeit quite pricey) a drobo, you can put multiple disks in there and hot swap to larger capacities as time goes on, yet your Mac will just see it as one big chunk of storage.
If, however you did want to split your library you'd need to select one disk to house the 'main' library. Regardless of what happens any content you buy from itunes (music, tv, movies, apps etc) will go into this 'main' library.
It's still possible to split the content you add manually but will require more management on your behalf.
To do it you'd have to stop iTunes moving new content into the media folder and stop it organsing for you. Then you connect all you externals with the files in the folder structure of your choosing then drag them/ add them via the menus into iTunes.
This way iTunes just keeps pointers to where the content is and doesn't move it.
Every time you add something new to the folder structure on an external you will need to add that manually.
This all works fine as long as you keep on top of it. But if you start to forget what's been added and what hasn't or you buy huge volumes of stuff from iTunes you're going to end up in a mess.
The bottom line is that a) it's possible b) the best solution is a single, unified hunk of strage as the library is designed to be in one place.
