Rob,
Like Ted, I exclusively use sink tips of 15' with my 2-hand rods. There are several brands available; however, I like the RIO's, which are available in sink rates of Intermediate, type 3, type 6, and type 8. These correspond to about 0.5"/sec, 3"/sec, 6"/sec, and 8"/sec respectfully.
The Windcutter line can be cast a very long way, as Ted pointed out George Cook has shown in some of his demonstrations. It takes good technique and practice to toss 100' with any line. The color change should be not more than about 4-6"" out the rod tip, with it being just inside the tipe being best for ease of picking up the belly of the line for the forward spey and energy transfer into the cast.
You still have your left hand available for holding loops of line with a 2-hand rod. Just because your left hand is being used on the lower spey grip for casting a 2-handed rod doesn't mean it cannot be used to hold loops of line. Hold loops of line that are 3-4 pulls of line with your forefinger of the left hand, keep the line from sliding out the rod with the forefinger of your right hand clamping it tightly against the rod grip, release the line from right forefinger by lifting your finger up just after applying your power stroke and fast rod stop when making the forward spey, and the line coils held in your left hand will flow right out the rod guides, just like with a single-hand rod. It takes a little practice to get the timing down for this and for using the index finger of your right hand for holding the line when forming the D Loop and making the forward spey; but with practice, it is pretty easy to make a cast of 75'-80' with the Windcutter.
The left hand pulling the rod's butt toward you in combination with the right hand pushing the rod's tip away from you puts tremendous energy into the cast and takes the place of the single-hand double-haul.
If you can, get some spey casting instruction from someone you know who is a decent spey caster, attend one of the various spey claves, or take a spey casting class. Getting some instruction will show you what I am talking about and greatly improve your technique while helping you prevent developing bad habits.
If none of three things mentioned in the last paragraph is possible, get yourself the RIO "INTERNATIONAL SPEY CASTING" video or better yet the DVD of it; the John and Amy Hazel spey casting video, Mel Krieger's spey casting video, or Derek Brown's spey casting video. I have a strong preference for RIO's because Simon G. is such a superb spey casting instructor. The $30.00 or so you will spend on a spey casting video or DVD would be money well spent and help you immensely.