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A deflected Cardan joint (U-joint) adds a 2 per revolution extra motion proportional to the square of the angular deflection. Two of them can cancel each other out but only if they are deflected to identical angles.
But the geometry of the rear suspension of BMWs is such that the Cardan joint angles are not anywhere near identical. That may be the reason the manufacturer doesn't give any recommendations on joint orientation.
I recall the rear angle goes thru a much greater excursion than the front. Hence it dominates.
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