I'm not sure how this works but I just completed a Melbourne race where two cars pitted on the same lap, yep when they came out of the pits and for the remainder of the race, my driver had one lap more wear the entire stint.
Thought it was a bit weird. Their pitbays were right next to each other. Not sure its great that my tyres are deemed to have done one lap more throughout every stint.
I have taken a screenie of the tyres amount of laps and the lap counter showing that the cars pitted on the same lap.