I have tried to set up 2 identities with Outlook ON THE SAME WINDOWS ACCOUNT and I could not do it.
You could try creating two separate Windows User Accounts, one for you, one for your wife (very easy to do in the Windows XP Control panel - if you have XP).
Then setup Outlook mail for yourself in one account and your wife in the other account. I have done this for my wife and my 2 children.
It is a good idea to have a separate Windows user accounts for each person.
Their personal files are stored in separate places, you can have your own "look and feel" (desktop image for example), your own browser favorites and so on.
If you give each Windows User Account a password there is also a certain amount of privacy (and less neeed for an Outllook password).
If you want help setting up a Windows User Account just ask.