Hello,
the Pages per sheet tool in the Toolbox and online chooses the layout by itself: with portrait pages, 2 pages per sheet are placed side by side on a landscape sheet. There is no option to change this there yet.
The PDF24 Reader (version 11.31 or later) can do it: open the PDF and use the tool Pages per sheet. Set Orientation to Portrait. The two pages are then placed one above the other on a portrait sheet, each scaled down to half the height.
The online tool: https://tools.pdf24.org/en/pages-per-sheet
An orientation option in the Pages per sheet tool would make this easier. Thank you for the suggestion, we will add it to our list.