I can't find the answer to this question anywhere - does anyone know?

I understand that a skill chance to hit and an artifact chance to hit don't stack (i.e. 50% skill chance to hit and 18% artifact chance to hit don't stack to become 68% chance to hit)

My question is: in a case such as this, does the game execute 1 or 2 chance to hit? i.e. execute only 50% chance (highest) or execute both 50% and 18% chances as separate attempts?