Two options really... (assuming you are a home user and not a business user on an exchange server)
Either setup a rule in outlook to automatically move all mails received on the second account into the other folder, if you need to send as the other person then you can enable the from option for when you type an email and select which person you want to send from each time.
Or setup a second profile for outlook so you get prompted each time you open outlook which profile you want to use, this will keep both accounts totally separate and is like having two copies of outlook.