The question sometimes comes up about using a 'W' valley. It's always a judgment call, in my opinion. It depends on roof slope(s), distance of run, volume expected from either direction. Only experience will provide the best solution.
However, when a very steep roof intersects a low-slope roof, a channeled valley is called for. I took a few scraps of left-over channeled valley for this display. The ridge can be seen to be 2" tall to help stop water crossover. The right edge is hemmed to catch any excess water on the low-slope side.
Here's an end view. Notice how the water from the steep slope will be inclined to stay in it's own trough. Notice that I did not hem the left side. If water gets there, you are in need of a rescue boat, not a roofer.
A top view of the channel/trough.
Here, I just took a small piece of metal to show how a 'W' valley would be shaped. It's a cut-away section, and would probable be hemmed on both sides, Absolutely hemmed on the low-slope side. The water will often jump clear over the W because it runs directly into the W.
A few more pictures can be found at Tinner's Photo Albums. Select the valley album to see one being installed on a faux slate roof. Feel free to look at all my albums, and feel free to ask any question you might have. I prefer 'educated customers' and don't mind telling the secrets of how your roof works as a complete system.
Albert's Roofing Repairs | Slate and Window Flashing Co., Rich., Va.