This is mostly a visual entry, but let me put into words some key factors for doing double pinch harmonics, just in case I left something out or didn't state them clear enough in the video.
If you are unsure of how to create a pinch harmonic or don't even know what it is, make sure to check out the video on Pinch Harmonics that I did. To do a double pinch harmonic is not easy, but it is possible with a little bit of play. The same principle applies. You pick a string and brush through it with your thumb, creating the harmonic, except this time you will be coordinating your thumb so that it is able to brush two string at once.
Here is some advice to get you started.
Hard to put into words. I know that I made that harder than it needed to be, but if you play around with this - you'll get it down. Best of luck!