First, if you are using a version of Outlook earlier than Outlook 2003, make
sure their combined size doesn't exceed about 1.5 GB or you may have touble
if you do combine them.  If you're using Outlook 2003 or later, you probably
don't have to worry about it.

If they're small enough, open them both with File>Open>Outlook Data File (or
Personal Folders File, depending on Outlook version - always post the
version).  Then for all NON-default folders (i.e, folders other than Inbox,
Outbox, Sent Items, Tasks, Calendar, Contacts, Journal, Deleted Items,
Drafts, and Notes), right-click the folder in one group of folders and drag
it to the root of the other group of folders.  Do NOT drag it to another
_folder_ in that group, drag it to the root.  The release the mouse button
and choose Copy or Move, as you see fit.

After copying the non-default folders, open each default folder in turn,
select the entire contents with CTRL-A (you'll have to display your Calendar
in a table view to do that), right-click the selection and drag it to the
corresponding folder in the other tree.  Release the mouse button and choose
Copy or Move, as you wish.

When you're finished, you can right-click each archive folder root and
choose Close to remove them from the folder list.