Splitting Up a Catalog - Best Practices?

I think this is the reason for making a clean break at a certain moment. There's no advantage IMO, and it is much harder, to move some images from Catalog A into Catalog B first, and then to move others later.

If you did want to do that, the means of doing that would be to highlight some images in A, Export as Catalog to a new intermediate Catalog X, go to B, Import from Another Catalog selecting to merge in the contents of X, review that it's worked, go back to A, remove (not delete from disk) the highlighted images, delete intermediate Catalog X.

As opposed to: in A higlight ALL the needed images, Export as Catalog B, remove those images from A.

If there is a difficulty in identifying which images should be highlighted for inclusion, then the priority should be IMO to progressively add metadata or labelling of some sort, until everything is suitably discriminated. Note that this process does not require physically moving any images into another folder. And then you can do your clean break all in one go.

There's no technical difficulty in having some images even within the same folder imported to one Catalog, and others to a different Catalog - though there can be confusion from some images being imported to both, especially when edits are being written out as external metadata, from either Catalog (or from both).

The methods described above all just involve re-referencing files that continue to live unaffected wherever they are.

If there is also a desire to separate the physical locations of files imported to Catalog A, from those imported to Catalog B - for example, if these will have different storage / backup requirements - then copies of the files, retaining all their relative arrangement into subfolders, can be made into a separate location as part of the Export as Catalog procedure. This is done with a checkbox in the Export as Catalog dialog, "Include Originals" (or "Negatives" iirc). It will then be these new file copies that get referenced by the newly made separate Catalog B, not the files that Catalog A is still referencing. Then the cleanup of these images out of Catalog A may include not just Removing theem, but also Deleting from Disk (to avoid doubled storage of these files going forward). Obviously this more radical option requires a lot more care, and so should definitely be supported by up-to-date full backups to other media of everything: image files and folders, and Catalog(s).

One rule on multiple catalogs; the image you're hoping to find is in the other one

Every time.

Thank you @John Waller , that is the simplest approach. I'd agree with your recommendation, that is what I would do ... but in some situations, separating personal photos from work photos makes sense to me (may not make sense to others). Many people have other reasons for splitting up a catalog, and in most cases (but not this one) I think that's a terrible idea for terrible reasons, and I will always try to dissuade them.

Because it's so many photos 500k+ this will be done over the course of a few months if not a few years.  So it wouldn't be done in one sitting.  

What I'm seeing as the stumbling block with that approach is how would I most efficiently get the next batch of wedding photos out of the old catalog and into the previously created wedding catalog? If it was possible to export from a catalog directly into a different catalog (without opening that catalog) it seems like it would efficient.  The ways I know of getting a batch of photos into a different catalog seem very time-consuming when you're sorting through hundreds of thousands of images a few thousand at a time.