You would just set it up like the other one, and choose which one you want as the default.
If you want to have separate Inboxes for each account, then I would recommend that you switch to Windows Live Mail (WLM). It makes sorting the email messages between the two (or more) easier to do. WLM supports Pop3, IMAP, HTTP website (EX: Hotmail) email, and is a lot more stable. WLM will automatically import Windows Mail messages, accounts, and settings into it. Contacts will have to be manually imported. (Tools, Contacts, File, Import, Address book for current Windows User.)