Mapping Center Answer:

If you're talking about joining two maps that show side-by-side areas, you would accomplish this by appending the data from one of the maps to the data for the other map.  To do this use either the Merge tool or the Append tool (read the help for both and decide which is best for you).  Once the data for both maps are in one dataset, then you can use the MXD that was originally used for that dataset and re-work the page layout to show the larger area, very likely needing to use a larger page size.

If you're wanting to combine the information from two maps of the same location into one map, then that's a much trickier proposition from a cartographic perspective.  Conventional cartographic wisdom would dictate keeping things simple--its often much faster and more effective to use two simple maps in place of one complex map.