You can’t access two libraries simultaneously, but you can utilize different libraries for different sessions, and it’s quite simple.
On Windows, simply hold down the SHIFT key while clicking through to the iTunes icon in your start menu. On Macs, use the ALT key. In other words, click shift, click the start button in your lower left hand corner (Windows), click the iTunes folder, and then click the iTunes icon to launch iTunes.
You will see a screen that allows you to choose which iTunes library you would like to use for your session.
Why would you do this? Multiple users on one machine (who don’t like the same kind of music). Different styles of music (I created a separate library for Xmas so all my Xmas music isn’t cluttering up my “normal” library). Etc.
The trick I’d really like to find is how to use multiple sources for iTunes files (one media folder on my machine, another on the network, another from an external drive, etc.). Currently, iTunes will only look in one place for media (you set that location in the edit > preferences > advanced window).
The only solution I’ve found is a program called PowerTunes, but it’s $20 and it seems like there should be an easier way to handle the problem.