Hi,
You can create a custom catalog Catalog Z_AB and then open catalog A, then drag the tiles which you need to copy from this catalog; you will get an option to select the catalog where you want to place this copied tile - here select your catalog Z_AB. In a similar way you can put required tiles from catalog A and catalog B in the custom catalog Z_AB.
Refer this document - Copying Tiles or Target Mappings - SAP Fiori Launchpad - SAP Library
Regards,
Saurabh