[abenedik]

The boolean operations split the triangles in the original mesh into smaller triangles as seen here - using your 2 meshes and a union operation:

   

This leads to some additional lines when detecting edges:


   

If you can create two meshes where their volume does not intersect, then you can use Ab3d.Utilities.MeshUtils.CombineMeshes method to combine two meshes into one - for example:

Code:

var boxMesh1 = new Ab3d.Meshes.BoxMesh3D(new Point3D(0, 0, 0), new Size3D(50, 50, 50), 1, 1, 1).Geometry;
           var boxMesh2 = new Ab3d.Meshes.BoxMesh3D(new Point3D(0, 28.25, 0), new Size3D(25, 7.5, 25), 1, 1, 1).Geometry;

var joinedBoxesMesh = Ab3d.Utilities.MeshUtils.CombineMeshes(boxMesh1, boxMesh2);

This produces the following: