Down tube and seat stay
The two are not directly related.  On a conventional diamond frame the downtube angle is dependent on the length of the top tube and seat tube, and, of course, the seat tube angle.  The length of the seat stay is dependent on the seat tube length and angle, and the length of the chain stay.

Obviously, the two "share" the seat tube length and angle (which is related in part to the overall bike "size"), but the length of the top tube is determined by the desired "reach" of the bike, while the length of the chain stay is determined by the desired overall wheelbase (longer for road bikes, eg), and the wheel size and desired clearance (eg, for fenders) between wheel and seat tube.

If you look at Google images for bikes you will see a wide variation in relative angles.  Usually the seat stay is at a steeper angle than the downtube.