since sketchup 8 there is a diffrence between solid groups and not solid groups. Now you can merge 2...

since sketchup 8 there is a diffrence between solid groups and not solid groups. Now you can merge 2 solid groups with the outer shell tool. already under - toolbars - solid groups! The .IGES plugin works only with solid groups and the export is a little bit buggy with smoth shaded Faces.