C++ SetWindowPos Problems adjusting window topping

Question

With the following code, I am trying to place window A at the top and then window B underneath window A. The Z sequence should be Top -> A -> B. But now I have a problem, for example, if a full screen window O appears, the Z sequence becomes Top -> O -> A -> B, and when I execute the code again, it becomes Top -> A -> O -> B, not Top -> A -> B -> O as I expected. This problem is probable, and I don't know the exact trigger reason, so I hope that someone can help me solve it. I hope someone can help me to solve this problem, thanks!

SetWindowPos(HwndA, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_NOACTIVATE);
SetWindowPos(HwndB, HwndA, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_NOACTIVATE);