Well Hi guys and I have got some questions here…
I can do Man on Flying Trapeze well with almost a perfect consistency after some practice but when I do Double or Nothing its either mounted on the inner string on on top of both strings !! I rarely mounted a perfect Double or Nothing and when I successfully did it my yoyo might be in a sloping position which force me to retract the yoyo back before i got time to see what happened actually.
the best i can say is practice.
move your hands to space the string out. don’t just keep them in one spot.
On the first wrap around your non throwhand index finger:
Make certain the string is at the base of the finger, as close to the hand as possible.
On the second wrap:
Make it as close to the end of the non throwhand index finger as possible
It takes some practice but it wont be long and you will hit it consistantly without even looking.
Hang in there!
Also, a better yoyo would help. Something with prolonged spin times, so you can correct your mistakes. Like the first guy said, practice practice practice.
heres a tip when i started to learn this mount first i got the two strings on your fingers then i started swinging the yoyo so i had enough momentum to get it in the trapeze
Practice the placement when the yoyo is dead it will help
Followed some advice I read somewhere and practiced the Houdini Mount to get the feel for how my hands should be spaced apart, etc. Then when moving to Double-or-Nothing, made extra sure the first wrap around non-throw-hand was as far in (away from fingertip) as I could to make space.
When practicing the Houdini Mount, you just KNOW that all unnecessary strings are out of the way, so you can land with confidence.
Moving to the D-o-N, when that first wrap is far down, you have similar confidence. If you feel that you’re not down far enough, meh… try to finish anyhow. Sometimes you’ll have the same problems (wrong string or two strings) but it’s still a practice throw. Can’t go wrong with practicing. 
Ok , this is how i learned it , keep two warp as far as possible since when you do it , the string’s too close then it’ll tilt . Try to space’s it out as far as possible but not too far
Oops! Hit the thank you button :o
Anyway! Trying only a portion of the Double or Nothing will help you with string placement. The Matrix video has another way you can try getting into a Double or Nothing as well. It’s all about string placement 
Nice ~ thx guys 
now i’d got it well ! Moving on to Split the Atom…
good luck!
All I can suggest is to practice and to have good hand I coordination. Hope it works out