Note also that the valley also sheds its water down the roof surface as well as opposed to into the gutter.
Gutter from one roof to another.
A steep roof covering a house has a larger area and collects more rain than a shallow roof on a house of the same size.
The one thing that pretty much everybody agrees on is that at least there should be a downspout to deliver the water to the lower roof and point it in the right direction.
As part of this system malarkey recommends against the drainage of one roof directly onto another especially because the lower roof often has less slope than the upper.
In this first picture one can see where the gutter has been cut off at the same pitch as the roof to let the gutter empty onto the roof surface.
Measure the area of each roof section to calculate gutter size.
The water is allowed to run on the roof surface and is collected by the lower gutters.