I had a guide in Colorado who tied the top fly on with a regular clinch knot but leave the tag end about a foot and half long. He would then tie the dropper nymph to that tag end. It worked fine.
I usually tie the heavier nymph to the tippet section then tie on the dropper nymph to the bend of that hook. I want the dropper or smaller fly to float a little freer with no weight. Seems to work.
As Gene said though, preventing tangles is more a function of the cast. Keep a slow open loop and you won't have as many problems.