I’ll try to get a video of this this weeknd. It’s like 1 am here so I don’t know why I’m still awake. 
What you do: When doing a normal ninja vanish, if you paused it in the middle of the trick, there would be a loop hanging over the string attached to your throwhand, right? Makes sense? For the Double GT Ninja Vanish, get that loop to form over your NTH wrist instead and land like you would for a regular.
If you want to land a reverse Double GT NJ (we gotta get a shorter name) then land the strings around the half of the yoyo closest to you. To land the regular Double GT NJ, you have to hit the left part of the loop into the gap, which will shoot to right part of the string around so that the loop ends up going around the half of the yoyo farter from you. (The same way that you would land a reverse ninja vanish). Also note that when you land either one, you will have string over your wrist and you should still have that string grabbed by your NTH fingers. Simply hold onto this string and drop the strings off of your wrist (and reverse if needed) and tada!
Hopefully that’s somewhat understandable at least until I get the video up. I don’t think it’s the same thing Kentaro did, but it’s hard to tell. Somebody could ask him I guess?
Also, in doing this, I figured out that you can do a ninja vanish over the top of your throwhand. It ends up not being a GT but whatever. It’s cool and hard to land.
To go along with this is Kentaro Kimura, who does one of these Double GTs at about 17 seconds: