Share page types across a multisite

So I’ve been trying to read up on this and while some seem to have had some success, I’m at somewhat of a loss on how to do it. We are building a (v9.4.7) multisite of two sibling domains, sharing a custom theme developed at the same time (so page types and templates are added as needed, the theme doesn’t have an existing blueprint for this).

The site building has been started with the theme on the primary domain and now that we want to start fleshing out the structures of each site, we’re finding we can’t use the page types created on the default site on the other one, that Concrete guided us through creating as a whole another type of site. The existing page types also can’t be duplicated on the secondary site’s side through copying or adding manually using the same handles, because those handles are taken by the originals.

I tried setting the secondary site’s siteTypeID to 1 in the Sites table as described here and that does enable adding pages of primary site’s types to the secondary site but that still doesn’t make the page types list properly for both sites in Dashboard > Pages & Themes > Page Types and also there’s the fear that hacks circumventing what the UI is guiding you to do like this might have some adverse effects down the line.

I also had the idea of duplicating the site types with prefixed handles on the secondary site’s side but I would like to avoid actual duplication if at all possible.

So, shared page types between two peas in a multisite pod. How do?