How do I get more than two active mailboxes in mail?

I just want to make sure I understand the question...you want to check 2 email addresses in mail? Or you want to versions of mail in your dock, each checking a different address?

If its two email addresses in mail, then open mail, setup your first email when the wizard starts.

once complete and working, you will go to 'Mail' in the top menu bar, and then 'Preferences'.

In Preferences under 'Accounts' tab, you'll see a left pane in the window that shows the email you just setup. Below that is a + and - sign. Click the + sign to add another mail account.