Totally concur with everything dshen has said above. Shinji's CGI is a good visualization aid: http://www.youtube.com/watch?v=JukMa...el_video_title
The best way to think of this is to walk! The natural way for us to walk is when we step forward with one foot, we swing the opposite hand forward right? Else we would be square-gaiting (in military terms). I also struggled with getting the coordination for the 2BK, and had to really concentrate hard on the timing. To get used to it, I forcefully did it in sequence (kick with right foot, then spear with left hand, etc) regardless of the outcome. This allowed me to at least get the right idea in my head, since it was the opposite to what my brain wanted me to do at the time. Took about a week of practice to get this right, and finally be able to do it comfortably with the right timing (almost simultaneous). It requires a lot of mental practice outside the pool also.
Good luck with your practice. I'd also add that the kick is more effectively done in whip-like manner (per Shinji) with emphasis on a foot/toe downward flick. Avoid a scissor kick... which I also had a tendency of doing at the start, more to do with over-rotating, but nevertheless a common problem that I've read about in this forum.