You can't make this work using the Active Directory or Exchange widgets - you have to use Outlook. There are some third party tools to do so out there, but if you can't use them or don't want to... The steps to do so are:

- On server (Active directory widget): grant user #2 full mailbox access to user #1
- On user #2 computer - add the additional user mailbox to the primary user account e.g. account settings, add additional mailbox
- Open subsidiary mailbox in Outlook, select calendar. Right click, properties.
- Permissions - add the desired user as a reviewer / editor / etc (choose your desired level)
- Click on calendars view, open shared calendar, verify you can open the shared calendar
- Go back to account settings, remove the subsidiary account
- On server remove full mailbox access
- Restart Outlook and verify you can still access the shared calendar

And you're all done.

Note if user #2 doesn't have Outlook then it's a moot point - OWA 2003 doesn't support multiple mailboxes or shared calendars, so there's no point in setting this up. You could also work around this by creating a new Outlook profile as user #1, setting the desired permissions, then removing the profile once done.