In addition to your Mac.com account, you can also set up an existing AOL Instant Messenger Account (AIM), another Mac.com account, or use your AOL screen name as a different account. No matter how many different accounts you have, though, only one can be active at any time. Every account can create a different Buddy List; if you use an existing account, it will pick up your Buddy List from that account.
To set up another iChat account: