I don’t think it’s necessary to use primary text frames for this.
Create a master spread with 2 pages. Add a (regular) text frame on each page of this spread. Do not thread them together, each should remain separate.
Go to page 1 of the document (or maybe page 2, if you want facing text). It should now be possible to place your text and autoflow it as usual, and because there is already a text frame on the master page, InDesign knows to auto flow the text into that frame only, so it skips a frame.
Does that work?