Point disapears if another point is overlaid

Hello,
I have two points on different layers.
The first has a green square as symbol and a size of 30 pixel. (bottom layer)
The second has a red diamond as symbol and a size of 15 pixel. (top layer)

If the two points have the same location (latitude and longitude) or are very close, I see only one point, the red point. The green square doesn’t overfill, although its size is twice as wide as the red point.

Is there a parameter of layer/shapefile/shape or point to not hide overlayed objects ?

Thanks in advance for your help

The solution is here :
Collision mode
tkCollisionMode