How to merge 2 one page PDF files
Open the two PDFs with Preview

Drag the thumbnails of the second PDF to the end of the first PDF but above the grey line (marked with the red arrow):





and finally export the resulting PDF to a third PDF.

If your first PDF contains just one page this doesn't work.

Workaround 1: Open your second PDF, then open the first PDF, drag the single page to the beginning of the second PDF and export the resulting PDF to a third PDF.

Workaround 2: Insert an empty second page at the end of the first page.





and drag the second PDF to the regular end of your first PDF



delete the empty page at the end and export the remaining pages to a new third PDF.