Dear MBoucher21,
thank you very much, your answer helped me a lot.
Steps 2 and 3 of your approach did not work well in my case, because the resulting line had many small intersections.
As an alternative, I used the "contour"-tool with the "dem1 minus dem2" result as input, "0" as base contour, and an interval high enough to make sure no other contours are drawn.
Kind regards,
David