How do you make a event based on collision whit one instance

What i mean is how you make a event or action based on if one sprite has collided whit another sprite and make it just affect that instance of that sprite and not the other instances of that sprite? You can't use the pick by UID instance for this if you couldn't tell.

When those 2 instances collide, what makes one of them the one and that other the other one?

I mean, if i set one on animationframe 1 and the other on animationframe 2, then i know wich one you want me to pick when you say: it is the one with animation frame 2.

How do you want me to identify them, would i want to make an example ?