Why you think that  can export as an spread? This is for another reason, to keep together this pages in indd.
1) Do this spreads as one page, and than export. (Page sizes in ID can be different).
2) And how you do export? In interactive pdf you can do export as the spreads by trick - http://indesignsecrets.com/interactive-pdf-from-indesign-cs5-always-has-spreads-on.php ,
In PDF for printer you can enable checkbox in settings for this.
Thanks for the (simple) trick it´s now a spread. Regarding export: I don´t really know why i´m using [ ] to export... That´s how i have always done it. Do you have any tips on other ways to export a 40 page portfolio?
As I understand your PDF is not interactive PDF. Print or Export all pages with single pages one time, than where you need spread as one page second time (with enabled spreads checkbox in settings of print/export), next step - combine their in Acrobat Pro.
I got it to work by simply changing the page size where i needed the spread to be. from one a4 to 2x a4 (horizontal).
As I say in my first post. Glad that you find how to solve problem