Depending what you mean, you can do this pretty easily.
Go to Control Panel - Mail.
Click Profiles. You'll have one as a default, and the default is that it'll just automatically start it up. If you'd like, you can click to be prompted for which profile you want to use, and then set up a different profile for each account.
I believe 12.6 will allow for multiple profiles in a single account, so you can "send as" and choose from a dropdown which account you want to send from. I don't think this will completely do what you want, but when people ask about multiple accounts, this is frequently what they're after.
You can set up delegation, and open the second account from within the first.
If you need further assistance with any of these, just ask.