How to overlap two layers of points with different size

I am new in QGIS. Here is my problem :

I have two layers of points. In these two layers, my points have different degrees or size :

1. For the layer 1, the point “a” has a size of 3, and the point “b” has a size of 8. The color of these two points is red.

2. For the layer 2, the point “a” has a size of 5, and the point “b” has a size of 2. The color of these two points is green.

Problem : When I overlap these two layers, then if I place the layer 1 first, we won’t be able to the see point “b” of the layer 2. Conversely, if I place the layer 2 first, we won’t be able to the see point “a” of the layer 1.

To fix the problem, I tried to go on : “Propriété de la couche” -> “Symbologie” -> “Rendu de couche” -> “Entité” and I choose “Multiplier”, to make points translucent, but the results are not satisfying me.

Is there a way to overlap these layers so that we place the points whose size is the smallest, above the points of larger size, despite their belonging to two different layers?