Remove the second mail account from this profile. Set up a separate Outlook
profile for the other person and tell OL to prompt for a profile on startup.
Or, if you're using a multi-user OS such as XP, set up another Windows
account/login for the other person so they get their own settings entirely,
including Outlook.
If these aren't actually different mail *accounts*, but are *aliases* or
something similar, this won't work. Get individual mail accounts for each
user.