For 1800 combo, you can pull it off by making full backward flip+handstand (start with handstand and constantly hold it till backflip is counted -then whole handstand is counted two times) +lucky landing with bounce and another handstand. The tricky part is good launch and getting bounce. Good luck ;P
For a 2x trick combo you have to make two tricks one after another without a pause. Just doing two tricks on the same jump won't do. Got it on 43 jump, now I want to forget the time I wasted on it. Maybe this post will help you finish this ealier.