Sorry, I think your question was overlooked when it was originally asked.
It is not possible to merge expositions, and I don't think it will be supported soon, as it would break some nice properties of expositions (for example, that pages links never change). However, you can use normal hyperlinks (https://guide.researchcatalogue.net/#hyperlinking) to create navigation paths for your readers between the two: if you do it well, the reader probably wouldn't even notice that there are two expositions. Also, in the table of contents, it is possible to supply (instead of selecting an internal page) an HTML hyperlink <a href="url"></a>. Using this method, it allows you to put "external" links in the TOC, for example to pages which are not part of another exposition. If you are not familiar with HTML's <a href=""> you may find this useful: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a