I'm a newish player and have been fiddling with this a bit today. The first thing I notices is that though it IS possible to jump dodge using JUMP + DODGE simultaneously, I find it waaaay to fickle and unreliable. After about 15 minutes of practicing I find the best way is to JUMP then after a very quick delay (1/4 second maybe) hit DODGE.
The other thing that helped is that I mapped DODGE to my middle mouse button (scroll wheel). I just press down and I dodge so I just need to hit the space bar, wait a split second then hit my middle mouse button and voila!!!