Technically, you cannot move photos from one catalog to another. You can copy the catalog information to another catalog.
Select the desired photos in Catalog #1, and then File->Export as Catalog, create a new catalog name for this export; then in Catalog #2, you can File->Import from Another Catalog and choose the exported catalog. This is a COPY of the catalog information, not a COPY of the photo. It is not a MOVE. Your photo remains where it was on one of your disks. Now the photo is in both catalogs. You could futher delete the photo from Catalog #1 (but not from the hard disk).
A much better idea, especially for beginners, is to use one catalog. This eliminates the extra work of moving/copying from one catalog to another, and has many other benefits as well, one of the primary benefits being simplicity.