Quite simple:
(1) Launch Aperture. Open the prefs and change the location of your library. Choose the folder you want your Library to be in. Don't worry, you don't actually need to choose a library file, it sufficient if you choose folder. Aperture will create one if necessary.
(2) Relaunch Aperture.
Once you have created both Libraries, you can open either one by double-clicking them. You could put the two library files into the Dock for easier access. You can also rename them, if you want.