Hi Pál,
in ancient (ars subtilior) notation there actually are noteheads with two stems (which may also be flagged differently), called "dragma". a picture search for "dragma ars subtilior" returned poor images; one not entirely useless is https://www.last.fm/music/Philippus+de+Caserta/+images/f82a66af9573ba3cf431b0b1986f07e8 (staff three, the black block between two red block in the first half of the staff); or see the youtube video https://www.youtube.com/watch?v=wd3ouxA9p-o
See also the examples given in Apel, the first of which being https://archive.org/details/notationofpolyph1953apel/page/392/mode/2up Best Lukas
