I have a scenario where I need to shingle only half a roof. It's a duplex, where there are two seperate owners. My roof is leaking at it needs to be replaced, and the other owner doesn't want to replace their half. It's shingled at about a 12x2 angle. Only part of our roofs touch. The very top which 42' by 14' on the front and on the back too, so we have 28' which share. How would I go about doing this?
We're currently considering either W Valley, or some kind of channeling

You will have to be more specific. Understand the 2:12 pitch but is it a shed roof, gable roof, hip roof and etc.. I assume you are saing that the roof demensions are 28'X42' and you are responsible for 14' X 42' of that roof. If you can repost showing and demensions as it relates to the style we would get a better idea of a solution. A picture would be a great help as well.

One reason is a 2-12 pitched roof is to low a pitch for shingles, if that's what it really is.
No idea what you mean by channel or the other term you used.

It appears that you have a shed roof (porch) as well thats shared. I would try to convence the other tennant to replace his half as well. It will cost less than if they have to have it done independently. I am not sure how your deed was prepared or the terms regarding maintenance but generally there is a clause that requires each other to make necessary repairs that affect the shared sections of the structure. You might check to see when it was last done and see exactly how much life is left (shingle life) and use that information to convence your neighbor of the need.

Generally when making the repairs to a roof without completely re-doing it requires weaving in the new shingles. If you would do this then you would have to weave the new section into their old roof. I guess you could split the roof and install a flashed curbing but it would have to be done to weatherproof both sides not just yours.