I would try another one with each of these documents.
- Create a new document with 2 pages.
- Name the master pages so, that their names are not conflicting with neither of these 2 document’s masters.
- Delete every style in that empty document, also any swatch
- No drag the pages of the first document into such an empty document.
- Do the same step 1 to 4 with the second document in a NEW empty document.
- Now you have 2 new documents. Take care that no conflicting master names appear.
- In each of these documents create in each style panel a folder and drag all used styles in that folder.
- Save both documents
- Try again to insert the pages of both documents.
Otherwise, it this does not work out, could you send me via private message the files or a link. I would check it with a Debugging Version of InDesign (InDesign DEV) I can use to find errors.