Learning the double thumb was tricky for me as well. Particularly because I started playing slap the wrong way (if that can be said about any technique that doesn't cause pain). I.E My thumb was bouncing off the string instead of sliding across and through it. Then I got the proper slap down, and started to understand how the double thumb worked. I tried to get it for awhile and failed repeatedly, so i gave up for a bit. Then one day I took another shot at it and got the motion. My problem now is that the gap between the end of my neck and the pickups isnt large enough to accomodate the double thumb double pop (24 frets). I don't really use the technique much but it is good to know. I find that first video interesting because I have always done what that person says not to do as far as popping goes. Why is his technique better? More streamlined, less movement? I guess it doesn't matter too much as I'm never really going to have to be able to do straight 32nd notes, and if it ever comes up, I can do the tap double thumb pop thing. Now I'm trying to find more ways to apply it musically.
Advice for you : If you get frustrated, just work on your regular slap technique more. The key to learning the double thumb is having a very smooth regular slap technique.
The important part is too make sure your thumb is going past the string you slapped to rest on the string below. If you get your thumb "underneath" the string too much when you try to pull back up for the double thumb it wont sound right and will probably hurt the side of your thumb. Your aiming to hit the same general area of your thumb, but on the opposite side.
I myself am still not 100% there. I revert back to my old form at high speeds, but sometimes I do it intentionally to get a heavier sound. I self taught myself the "incorrect" form because it doesn't cause a clicking sound on my bass the way it would on many other basses. The way the neck / pickups are set up and the style of pickups make it so the strings dont hit the pickups when I slap that way.
Hope that helps in some way, I only recently picked this up myself so it is very attainable! Good luck!