What is the name of a roof section between converging valleys?
tl;dr: It probably doesn't have a formal name because it's not special.
You rectify the problem of a horizontal valley by making it into two non-horizontal (sloped) valleys. You accomplish this by raising one end of the valley, which has the effect of widening the valley floor and sloping it. This, in turn, results in two valleys.
Call it a diverter if you like, but it really doesn't divert anything--the water ends up at the same place it would have, more or less. It's just another roof plane, really, equivalent to any other roof plane. It just happens to have converging valleys flanking it.
The same situation occurs between two dormers, for example:

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