Are those pages a spread in your InDesign document? The option "As spread" does not mean "any two pages, side by side".
If they are, then your InDesign is malfunctioning (and you may want to add some more detail then).
If they are not, either change your document to actually create a spread made up of these two pages, or, if you don't want to change the document, create a new one with proper spreads and place the two pages as imported documents, then export from this document.
Try this trick if you have weird page numbering in your document:
Go to the Pages panel and select your spread(s). Click the Pages panel menu and click Print Spread. Copy what you see (i.e., "Letter V:2-Letter V:3") and cancel out, because you can't Export to PDF from the Pages panel—you can only Print.
Export to PDF. Paste into the Range (i.e., "Letter V:2-Letter V:3"). Select Spreads. You will get exactly what you want when you export to PDF.
(For a simpler document, you may not need this.)