Jed,
I am not sure how to open multiple catalogs. I seem not to be able to find a command that says "open catalog X"
Catalog->Catalog Management
All of the catalogs listed are known to Photo Mechanic Plus and are open. Whether they're active for search or for modification is determined by the two columns of check boxes.
If a catalog is not listed, use the "Locate Existing Catalog..." button and choose the catalog folder you wish to open.
But unless you really need more than one catalog, it is probably not worth the extra complexity for most users. You'll have to be keenly aware of which catalogs you want items added into and which ones you want active for searching. Large catalogs are not usually a problem so don't worry about size before delving into multiple catalogs.
-Kirk