Make them into two separate files, save each as a PDF, and then use Adobe Acrobat to make them into a single file.
PS does not do multiple pages.
Photoshop CS does do pdf presentations, however, which is sort of the same thing…
Heather,
Photoshop’s PDF Presentation feature does provide an easy way to create multi-page PDFs from multiple source images, however as Tim indicated, it is only available in Photoshop CS, not in Photoshop 6.
Paul