In the case of the green airplane splitting at the wings, you should add another group, so that the wing bones are at right angles to the intermediate group, which you will place between the front and rear bones. Thus you will have a nose-cone bone, a fuselage bone (with wingbones on either side), and a tailbone. Using the "weld identical vertices" command won't work correctly if any of the group boundary vertices are not identical. So you must double-check all group boundary vertices in Rhino prior to exporting, using a magnification of 16X (1600 %) or more. If Rhino's vertex modeller allows you to enter numeric values for vertex coordinates, then copy-and-paste the same values for boundary vertices.