How to sew the Corners of sofa covers by decorative stitching machine

The challenge in sewing production of leather sofa covers is to stitch corners of covers, especially when use double needle sewing machine. 

But still possible, this video show how to sew the Corners of sofa covers by Hightex 7700P double needle decorative stitching machine. 

The sewing speed is very slow, because the worker is just a beginner. For skilled worker, the speed should be triple.

http://www.cowboysew.com/thick-thread-decorative-stitch.htm