First, never "print to PDF." That's a crap option for things like Word or other apps that can only export PDF using a plug-in print driver.
When you Export to PDF, selecting 'print as Spreads' should create "solid" unified 2-up pages; end of problem.
If you're exporting to PDF, using Spreads, and getting indivdual pages... I can't say from what you've provided where the problem is. Are you setting up "Export to PDF" and then using "Print to PDF" to actually generate the output? Because — if it's not clear already — those are two separate processes, and to say only one should be used is a way to say the other option shouldn't even exist. 🙂
One more option that doesn't seem to be part of this is that if you export a PDF as individual pages, but set it to "Display 2-up," that's just a guideline to the eventual PDF reader as to how to display the pages... and besides being able to override it in Acrobat Pro and Reader, many third-party readers (Foxit, etc. and those built into browsers) don't support the viewing options. Never blame any problem on a PDF or a workflow unless you're proofing the result in real Adobe Acrobat.