I wouldnt say that Ancestry DNA (or any other company) has the ability to "find" a relationship. The results "suggest" a relationship, but it is a range in which a number of relationships might fit. For example, shared dna of 50% could predict a mother, father, or sibling. a 25% shared dna amount might suggest grandparents, aunt or uncles, half-siblings, or double cousins.
In the instance you describe, if I am understanding you correctly, you are wondering about the percentage shared by a child produced by two siblings, correct? As I understand it, the amount of shared dna would still be 50%, so the child would be within the range of a child of both parents (siblings), but there would be a greater amount of dna shared with the grandparents (siblings' parents) than normally would be expected of a grandparent/grandchild of 25% shared. I've never been good at math, but I think the shared dna percentage between the grandparents and grandchild would be that of parents...50%.
I could be totally off base on this one, so I would hope someone else might add to it. Lest anyone think this is too "out there", one of my matches grandfather happens to be the son of two siblings.