I have been using Scientific Anglers Mastery Steelhead Taper fly line on both my 9'6" 6 wt and 10' 8wt this season. The SA Steelhead taper has a longer belly than traditional Weight Forward lines, enabling longer mends and advertised roll casts to 70'. I initially started casting the rods chiefly with roll casts and realized that I was missing out an a whole array of techniques that could enable me to cast further and with less effort, meanwhile keeping my fly in the water longer. I also realized the limitations that a roll cast presents when wanting to change the direction of a cast. I started utilizing two handed casting techniques and opened up a whole new section of water previously unreachable, revolutionizing the way I fish. I unknowingly adopted the skagit style of casting. Now I'm known down at the river as the guy who "roll casts" to carp.
On my 9 wt, I typically fish my Rio Clouser line with it. It has an oversize front taper on it, great for delivering heavy, bulky flies. I am able to utilize two handed casts with it just fine. The extra weight (grains) of the 9 wt line allows me to cast further than my 8 or 6 weight rods. The key to roll casting and skagit-style casting is to keep in mind that the line on the water's surface serves as an anchor point, which is utilized to load your rod. It is the interaction between the surface tension of the line on the water's surface which will affect your ability to perform the cast. In essence, most two handed spey/skagit casts, and their derivative forms, are based off of the roll cast. Master the roll cast, and you are ready to move on to learning two handed casting techniques.
One thing to keep in mind is that many of the newer blanks that high performance switch and spey/skagit rods are wrapped with are often reinforced, to handle the multi-directional torque of loading the rod that two handed casts produce. Using a heavy grained line to perform two handed casts on a single handed rod may add stress to the blank that it was not designed for. Cast at your own risk. Another thing is to make sure your ferrules are TIGHT. This could have been one reason my Scott broke on me this summer. I believe the ferrules may have come slightly loose from performing two handed casts. When I hooked up with that carp, if the ferrules were loose, that would have likely resulted in a broken rod. From what I have heard from other anglers, loose ferrules is quite common when performing two handed casts. Use of ferrule wax may be a cheap investment to keep your rod sections from separating and the potential catastrophic loss of a rod.
Coming up: The applications, fundamentals and differences between the spey and skagit styles of two handed casting.