Funny story actually, I've been talking to both my friend and sister about the Aasimar race, and we thought of a funny idea though not so mechanically concept for the Aasimar/Tiefling hybrid babies.
Aasimar with Scion of Humanity and Racial Heritage (Tiefling)
or
Tiefling with Pass for Human and Racial Heritage (Aasimar)
Aasimar could grab some extra Natural Armor with the Armor of the Pit, while Tiefling could get some interesting feats themselves. Actually, in light of that... a Chaotic Neutral Tiefling worshiping a Neutral deity with Racial Heritage (Aasimar), and the Channel Force would be an interesting concept.
Is push good enough to justify it though?
Hm.... then how would one make a crossbreed between Aasimar and Tiefling? They are both human ancestry technically.
~Edit~
Maybe Human with Crossblooded Sorcerer Demonic/Celestial?
Or maybe Aasimar with Demonic or Tiefling with Celestial?