MTL,
Here's what it looks like to me: basically, you want to create a risky 2-shot hole and a somewhat safer 3-shot hole that live on top of each other. In my experience, this design concept is one of the hardest you can ever tackle. There are just so many things that have to be right, and every time you fix one problem, you create another. In fact, my last article in the PDGA magazine was on this very topic.
In that article, I make the case that the type of two-shot hole you want will generally have a dogleg and so will be V-shaped. Three-shot holes will have two doglegs and will be U-shaped or Z-shaped. The fairway you're working with is dead straight, so you're trying to put an "I" on top of an "I." That makes a hard job even harder.
Chuck and Denny make good points about the value of diagonal OB. But you've got bigger problems. Bottom line:
1. You can make a good two-shot hole for 1000+ rated players in the 750-850' range. The low ceiling and stream you propose make the approach risky enough that someone's got to want it bad to try it, but it's not so hard that it's a matter of desperation. How long the hole should be depends on how hard that approach really is. Based on everything you've told us, I'd guess that 850' is too long to yield many eagles, and you very much want it to produce a decent percentage of eagles. Otherwise, no thinking golfer will be tempted.
2. Your biggest problem is the resulting three-shot hole. Even with the tee side of the stream at 150' short of an 800' pin, you'll have top players looking to go about 600' on their first two shots. That leaves them a safe 50' short of the "water," and it gives them 200' to the pin. It's dead straight and dead flat. And with a width of over 100', it might as well be wide open. Your top players can throw a putter twice and be right where they want to be, ten times out of ten. The three-shot hole is not fun or challenging. And not even installing the stream on a diagonal can save you.
So if someone needs to make up a stroke, he can go for it. If he's happy where he is, he plays what's probably the most boring hole in your entire tournament, and I know that's not what you're looking for.
The best case would be that the leader sees the second-place guy going for it and decides to risk going for it, too. With that in mind, I guess I would want anyone hitting that branch to go into the water. I just don't think that's a likely scenario, and you'll still have a bunch of other players taking the safe, no-fun option.
So I hate to say it, but unless I'm missing something, I think you need to go back to the drawing board. You just don't have enough to work with there. If it helps, it looks like you probably have better places to combine two holes.
Hope that makes sense. If there's something I misunderstood, please let me know.
Thanks,
John