The command to write tag information into the photos is File > Write Tag Info to Files. A detailed explanation of the procedure follows. Of course, you should perform the backup command on both catalogs before starting.
First, decide which catalog will be the source and which will be the destination. I would suggest you choose the catalog with the smaller number of total files as your source. By using the smaller catalog as the source, you'll presumably have less work to do when mapping tags in the destination catalog, but of course you do not have to use the smaller catalog as the source.
Secondly, decide if you want to copy the tag hierarchy from the source catalog to the destination. If you only need the tag names and not the hierarchy or tag notes, you do not need to do this step. If you do want to keep your tag hierarchy and/or tag notes, go to the Keyword tags panel in the source catalog, click the + icon, and choose Save Keyword Tags To File. You will be asked if you want to export all keyword tags, or only specific categories of keywords. Make your selections, name your file and save it as an XML file.
Before exiting the source catalog, select all the photos in the catalog (Ctrl-A is the shortcut for select all), then go to File and choose Write Tag Info to Files. (I think it's a good idea to perform this command periodically on all photos in any of your catalogs. It can't hurt to have the tags preserved in the image files.) It may take some time for Elements to write the tag info if you have a lot of files in the catalog.
Next, think about the actual folder locations of the photos in your source catalog. If the photos are all in one place, you're good to go. But if the photos from the source catalog are scattered about, you may find it helpful to first export them all to a single folder so you only need to point the destination catalog to a single location. Do this by selecting all photos, then choosing File > Export Photos > As New Files. Make sure you select original format and original name in the Export files dialog box.
At this point, you can close the source catalog and open the destination catalog. If you saved your tag hierarchy to a file, first import the XML file you saved previously by clicking the + icon in the keyword tag panel and choosing From File. Then navigate to the XML file and select it.
Finally, use the "Get Photos" command to point Elements to the source photos and import them into the catalog. If the files are scattered about and you did not export them to a single folder, you can scan the entire computer for photos.
Don't worry about importing duplicates if you have photos in both catalogs. Elements will skip any photos it already has in the current catalog. When the photos are added to the catalog, the Organizer will recognize the tags stored in the images and you will be asked how you want to map the new tags to your existing tags. Your captions will also carry over to the new catalog as these are written to the files automatically.
Note, however, that you may lose some features from the source catalog. For instance, stacks, version sets, and collections (called Albums in version 6+) will not carry over. You will need to recreate these. Versions sets cannot be recreated; you can reconfigure them as stacks, but you will loose the Organizer's special handling of version sets. If you use daily notes in the Date View, you will need to manually copy these to the destination catalog.
When you are confident that everything is copied into the destination catalog and you've recreated any stacks and collections, be sure to create a new backup which includes your changes.
