You can't layer two documents but you could use the Place and Link functionality in InDesign (Edit > Place and Link). This would allow you to have one document that contained the master text and then place that text in another document. When you modify the text in the master file, any file that is using text linked to that master file will indicate that it needs to be updated and can bring in the modified text. Another option would be to export the text as an InCopy story (.icml). Choose Edit > InCopy > Export > Selection, then place that story in both InDesign documents. You'll have to use the Assignments panel to then check the text out for editing but it would achieve a similar result. The first options would be the simpler of the two.