when you are stiching them together, do not grab a lot of material in your whipstitch, just catching a few threads should be enough and will show a lot less. also, when you go through the fabrics, make that as perpendicular as you can, then when you bring it back around the top, make that one at the angle you need to get to the next position. ....i'm not sure if that made sense....let me know if not, and maybe I can try again with pictures.
And pull those whip stitches tighter, they look a little loose.
Those should help your stitches disappear more