Is there any options to have a double arrow headed mark (i.e. arrow head on start and not only on end side)? I had a look at the code on github but it doesn't look like an option. I wanted to fake it by drawing in 2 opposite directions but it does of course not work well with the bend option (I am using it with Plot.geo)

You can use a negative bend on the opposite direction (bend: true defaults to 22.5°, so use bend: -22.5); see https://observablehq.com/@recifs/double-arrowhead-plot for an example.