change texture of specific material in object that contains 2 materials?

This is one way to do it, you will have to know the name of the material and name of the texture property.

 Material foundMaterial = null;
 foreach(Material mat in renderer.materials)
 {
     if(mat.name == "my-material")
     {
         foundMaterial = mat;
     }
 }
         
 if(foundMaterial)
 {
     foundMaterial.SetTexture("propertyName", myTexture);    
 }

To find the property name of your texture, look at your shader script. Check highlighted.