Try this for a tandem fly rig:
1. 3' leader butt of 20# flourocarbon with a 2' tippet of 10# flourocarbon.
2. Tie the trailer fly (I like a large unweighted deceiver) with a no-slip mono knot to the tippet.
3. In the butt section, about 6" ahead of the tippet tie a dropper knot (Practical Fishing Knots II, Krey & Sosin)
4. Contruct the dropper of 10# flourcarbon if you expect snags (you can break it off if necessary), 20# if you don't(stiffer dropper). I like a weighted fly for the dropper as it tends to fall away below the tippet without fouling the main leader. Keep the dropper to about 10" or so, tie a perfection loop into the end of the dropper and make a loop to loop connection with the dropper loop in the butt section. It also helps if the clauser or other weighted fly is tied on with an improved cinch knot as opposed to a loop knot.
This system has a number of advantages and rarely fouls. While the leader may take a few minutes to set up initially, it is quite easy to change fly combinations or tippets. The loop to loop droppper connection and the weighted fly below provided a "stiff" connection perpendicular to the leader which minimizes fouling. This rig also preserves the action of both flies whereas tying into the bend of the lead fly tends to "kill" it. I have also tried "Zap-a-Gapping" the dropper knot as it is the weak link in the system. I haven't decided whether or not the glue is good for the flourocarbon material but I have landed some fish in the 15 to 20 pound class on this rig.
Give it a try. It works!