People on QB are always presenting new ideas. Way to go!
If the quilt is in competition at a show, judges don't approve of zigzag on the back. What would they say on an art quilt? Don't know, since art quilts break rules.
I like the look of the double needle.
The width of the double needle to use is determined by the DSM. Some will take up to 9mm, while others only take 4mm.
One other thing is that double needles cost more. So when I use them, I'm very careful to put little stress on them.