How to scale a long LibreOffice Calc sheet to fit on two pages when exporting to PDF?

You can't use the page preview options to scale the PDF output. Output scaling has to be done using the Sheet properties in the Page Style dialogue (Select the Menu Format -> Page... -> Sheet Tab).

There, you have different options to make the Calc output fit to a certain number of pages. In the following screenshot, the Scale options are highlighted, especially the Scaling mode ComboBox:

To scale the PDF output to two pages, just select the third option (Fit print range(s) on number of pages) from the Scaling Mode ComboBox and set the page number to 2. This should give a PDF output scaled to two pages: