i dont think thats right. ADH and aldosterone serve to increase blood pressure by increasing water absorption. Since they both increase blood volume, osmolarity of the blood goes down (smaller ratio of dissolved stuff to blood fluid).
Although aldosterone causes increased absorption of Na+, the blood pressure was low to begin with, which means that the blood volume is low, which means that the concentration of stuff in the blood (compared to how much fluid is in the blood) is high. so, aldosterone is secreted, Na+ is absorbed, water absorbed too, and the blood volume is higher, which makes the osmolarity of the blood lower.
so, both, ADH and aldosterone serve to decrease blood osmolarity. Does this logic seem reasonable?