Rhino refuses to merge the most primitive of surfaces

I can’t merge surfaces no matter what I do. I created a new empty file and placed two box meshes right at the edge of each other. Whenever I try to merge them, I get the “edges are too far apart to be merged”. If I increase the tolerance, say by 100, it does merge them but it completely blends them together instead of neatly eliminating the shared edge. Lower tolerance of 1 or 10 or .1 changes nothing, keeps giving me a “edges too far” error.

Hello - please see Help on MergeSrf to get an idea what that is meant to do - meantime, BooleanUnion ought to work on your boxes.