While using Apple's products is simpler if you only have one Apple ID, there's nothing stopping you from creating another Apple account if you already have one. Apple uses your email account as a unique identifier in its system, so you can't create two accounts with the same email address. You can, however, create another Apple ID using a different email account.
Use a Different Email Account
The only thing you need to create a new Apple ID is a different email account from the one associated with your current Apple account. Google, Yahoo, and Microsoft all offer free email accounts you can sign up for, or you can set up a new one with your Internet service provider. Once you have a new email, you can use it in Apple's normal registration process to create another Apple account.
Apple ID Signup
To create a new Apple ID, go to the Apple ID website (link in Resources) and fill out the sign up form with your personal information. Use a different email address than the one you use for your current Apple account. Other than using a different email account, providing the same personal information you did when you created your original Apple ID will help ensure you can recover your new account's password if you forget it. Click the "Create Apple ID" button at the bottom of the page when you're done to create the new account.