Although the family instance variables are used and apply to all family members, they can be set separately. So if you have a family instance variable BulletDam, it will appear on all bullets as a variable but Bullet A can be set to 10 and Bullet B can be set to 20.
So you set the individual bullets A and B up on a sheet, where they have their default values for BulletDam. Then when you do the general event if Family Bullet is overlapping Family Enemy, subtract Family Bullet.BulletDam from Family Enemy, in just one event you can set up something where any bullet overlapping any enemy will subtract its own damage from the enemy. I think that's what you wanted, in just one event.