Anyone know a general rule of thumb for maximum width of a dormer valley rafter framing method.

I have 2- 2 foot offset gables that are 16lf in width 12/12 pitch 2x10s and a main gable 9/12 34lf width. I am thinking about doubling the rafters about 7lf apart in order to make a closet in the offset but I don’t have any bearing directly under where the valley beam will sit. I was wondering if this was too long a span.