Which U/M Coverage Do You Apply First?

If I get hit by an uninsured motorist and have bills up to $75K, and I have a policy with $25K, and I live with my parents, who have three policies with U/M of 25/50, 50/100, and 25/50, how can I stack my U/M coverage in Georgia? Do my parents' policies pay a prorated amount of the remaining $50K? Or is there another way?