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Don’t do it. It’ll look shit.
Make the shelves to the depth of one of the chimney breasts (ideally the shallowest) and commit to it. If they are straight they will look fine.I made these to go round our very shallow and not-perpendicular chimney breast. The bookshelf on the right lines up only with the boxed in section and the window. The one on the right lines up with nothing. But because they are straight they seem to fit just fine.
Let's hope it's just because of arbitrary measuring, as opposed to something more expensive...
Do the shelves need to be rectangles, or will any old irregular quadrilateral do?
You could stud out the shallower chimney breast if you wanted them equal.