How to tell my dads I want to know which one of them is my biological dad

I have two dads, Dad, and Pop. I have no idea which one of them is my biological father. They had embryos implanted into a surrogate. One with dads sperm and one with Pops. I'm the only one that took. They never got a paternity test because they are both my fathers,which is true.... Every time I ask about it they just say they're both my dads. How can I ask them to do a paternity test?