Ian,
I am not generally a fan of the side-by-side fairway holes, because I believe the strategy is often backwards from what it should be ... the guy who can make a long carry gets the best of them, while the shorter hitters are stuck with a second shot which emphasizes their weaknesses.
I have designed a few, and I try to do it the other way around. At the project we're currently building in Washington, we have a split-fairway hole, the par-5 15th. It's 500 yards and downwind so it should be fairly simple to reach in two, but to do so we are making you carry a small creek about 230-240 yards on the direct line to the green, and the fairway in that zone is relatively narrow. You can also play the hole longer out to the left, but I deliberately cut the fairway off at about 285 yards on this line [where the creek runs into a lake]. The short hitter plays it as a simple 3-shot hole with wide landing areas and a small carry over the creek for his second; the long hitter has to go for the difficult carry and flirt with the stream if he wants to be rewarded for his length.
Sorry I don't know how to provide a diagram here.