I fish two flies pretty often. I make no warranties or representations as to the science of my technique or the effect on know strength, etc., but here goes:
I attach my leader to the last tippet section with a double surgeon's knot, and leave a tag end of 4" or 5" hanging off the knot. That's where I tie on the 2nd fly. As long as the point fly is heavier than the dropper [ummm...terminology confusion: by "point" I mean the fly at the end of the line, by "dropper" I mean the one dangling from surgeon's knot] I don't have much difficulty casting. The dropper will sometimes twist around the leader and give you a good wind knot, but not too often.
Since I like to use a relatively weightless fly at the dropper, it's generally a Ray's fly. This also lets me experiment with my main fly a bit while remaining confident that there is at least one proven fish catcher on the line.