Depending on the settings you have in Outlook regarding if a copy of emails are left on your email server, essentially once you download the file into Outlook the only way to share that is a little tricky.

Generally when I have clients who want this functionality I suggest the use a web-based email client instead of Outlook. Most internet providers have a web-based client that allows you to access your emails from any web browser including Safari on your iPhone. You can then see what emails have been opened, what have been flagged and it would allow you to login simultaneously with another user.

This would provide the easiest way to achieve what you are after.