Ahh now i know what you mean with double jump. Its just that there is already the term 'double jump' which means what i did descripe.
What you mean is more or less bunnyhopping. And if you mean the jumps on darkzone over those two gaps i think i can help you. The first one by the mortar is just timing. Just jump (+forward and +jump) and while in the air release and repress +jump that should do the trick. You have to hit the right spot because the second jump has just the right width.
Thats how you gain speed in Nexuiz 2.0. Just jumping in a stright line, just +forward pressed. That funny stuff like strafe jumping might even slow you down..
The other jump over the rocketlauncher is very similar. Just jump (+forward and +jump) while in the air release and repress +jump, release +forward and press +straferight/left instead and turn your mouse into that direction.
Oh and i noticed the jumps on skyway can be pretty easy if done right. The previous version was all three jumps as single demos as i could not do them all togehter without mistakes

. So here is a demo off all the three jumps together:
*edit* better use that demo pack:
http://www.forums.alientrap.local/viewtopi ... =6709#6709