I hear you Jason, I can cast with a single fly all day, be it small or big. When I put on a second or third I get tangled farely often. That is unless I am in a drift boat and am able to do just a "roll cast". Basically once the line is past you, you let the water load your road and with a quick flick of your wrist you put it all back up stream in front of you and mend as it goes downstream.
This is great in boats where you are basically right on the run and only have 6-10 feet of fly line out and then tippet.
I have just been reading a book called "1001 Fly Fishing Tips" and one thing they suggested was bigger loops on your cast which, bring your arm back slightly away from your body and then on your forward cast bring your rod straight over your shoulder, if that makes sense. I messed around with it today with a hopper and a prince nymph and felt it helped.
From the book;
"Be prepared for the worst. Though you can minimize them, tangles are a fact of life with tandem-fly rigs. Carry extra leader and tipper to rebuild rigs, build the mental stamina to patiently pick out a wind knot, and learn to recognize the times when you must the flies off and start over."
I am not sure you will ever get it 100% with a tandem setup but I also feel 6 inches is a bit short and agree with birddog's recommendation of longer sections between flies personally 12-18 or so.
I would also be cautious of very different size and weights of flies on a tandem setup, but that is just a guess.
Hope this helps a little, I am by no means a pro with it so just relaying what I have heard.