Create a second iTunes library?

I do have an answer for you, but I'd like to suggest an alternative. I would use the "grouping" tag field and tag your popular music as "popular music" (or "popular" whatever works) and your classical as "classical" and then create Smart Playlists using "If 'Grouping' 'Is' 'Classical'" and "If 'Grouping' 'Is' 'Popular'" to separate the two.

If you're set on creating two libraries, when you click on iTunes to launch it, hold down the option key (if you're on Windows, I'm not sure what key would be.. my guess is Alt) and you'll be presented with Quit, Create Library, and Choose Library. You can use that to create a second Library. iTunes will launch the most recently closed library but if you want to go to your other one then just hold down option and select Choose Library, then select the parent folder of your second library.

The only reason I suggest using smart playlists is for syncing iPods because you can only have it sync'd with one library at a time... that and I managed an audio and video library as separate ones for over a year before finally merging them after being frustrated with having to switch back and forth all the time.

 

It certainly helps. I used them when I was re-encoding my Library and had "old" and "new" Smart playlists. I never actually would sync the entire playlist but I at least knew that when making playlists I could start by only looking at the "new" playlist rather than having to go through the whole library and try to remember what was old and was new.
I've since forced myself to try and use the smart playlists more often and because of that have actually learned quite a few things that have helped keep things organized.. I definitely recommend it to having two separate libraries - it's way more practical when it comes to syncing to ipods/appletv/etc.