Share Outlook calendars between two separate domains using exchange 2003

I need to be able to have users share calendar information from two different domains.  The situation is our company was recently acquired by another.  We are on two different domains - no trust exists at all.  We need to be able to have some users share calendars between the two domains.  Essentially, we have a couple users who have email accounts on both domains for simplicity but we really just want these users to access only one email account on either domain but yet have both domain calendars synced.  Any suggestions would be helpful.