I personally love double kongs...but don't really think they're that practical. If you can dive kong it, there's no real need to double kong. It took me about 2 months to get it down nice. My progression for double kongs (still working on it)....
(all this in a gymnastics gym with foam blocks)
I started off on something easy enough to dive kong over 7ft ish long (not two separate objects, just one long one), little bit higher then waist height. Just did that until I was comfortable with it/ could land it nicely. Then I moved onto a traveling kong to get used to orienting my body in the air the way I'd need to for the double kong.
Before moving onto the actual double I practiced being in a handstand on the object I was trying to kong over and dropping into the standard kong landing since that's basically how you're gonna be landing anyway (except with forward momentum). I mean you're basically at a 75 degree angle going over it, unless you're really pushing the length of your kong...which you shouldn't until you get it down good.
Still working with the 7ft long obstacle I kind of just went for a kong to dive roll just in case my shoulders/arms/back couldn't take the load. Felt it out, adjusted my take off and went for it. I got it on my 1st try...but had to roll the landing cause I came in too fast. I played around with where I felt most comfortable with my hand placement/take off before I actually tried to double kong two separate objects. I'm working on different levels now just to see what I can do, low to high is tough but do-able, high to low still scares me a bit. I'm also currently working up to a double kong on rails (about 5ft apart)...but not enough confidence in my hand placement yet.
GL and train safely