This is a precision modelling question which I'm hoping has a somewhat updated answer. Historical suggestions either seemed too complex or haven't worked as expected.
These objects are identical in width and height, and consist of only outer and top faces.
The intuitive approach would be to have the knife snap to the intersections of the two objects ( in top view ), and draw the cut line between these snap points. This doesn't appear to be available in 2.92.
Is there a quick and easy way to accomplish this? Grid snap is of no use, as shown in the maximum close up of one of the intersection points. I've tried all manner of other snap options but I'm either doing it wrong or they refuse to activate.