Hi niko, the main thing is the edges have to be tangent / smooth to one another in order to be mergeable. If they make a sharp corner where they touch then they can't get merged into a single edge.

Also they have to be the only 2 edges that touch at that common point, there can't be other edges hanging off of the same point going perpendicular to them or anything like that.

There are also a couple of situations that can't be merged, like if the edges are crossing over a surface pole point (where all the surface control points are collapsed together) like the pole of a sphere.

Also if the edges are part of a seam edge of a closed surface, those won't merge currently but I think that's something I can fix up to be allowed.

- Michael