Ah, thx! That indeed does work in both PolyEd and TriEd. However, of course its a bit cumbersome having to delete the faces first. In TriEd mode you are likely having to flip at least one edge on the left or right side. In PolyEd you even have to insert edges on both sides to be able to delete the faces. In Unity ProBuilder as well as Blender this kind of thing happens automatically as it is clear which triangles need to be deleted during the weld.