W1ckd,

An archived PST is not synchronized with your exchange server as it is local. As you already are aware the archive cannot be opened by two clients at the same time. This is intended by Microsoft and could cause permanent damage to the file and your Outlook program.

A good work around would be to copy the archive and rename it. This way you can open the copy in the other program.

Note: if you archive you mail to one .PST file and this in theory is a back up leaving a copy still synchronized on the server, it is possible to archive it to the copy as well.

Hope this helps.