Upper Planetouched + Lower Planetouched = ?

Personally I'd just rule 'em sterile to each other.

Well there are several possibilities. I'll try to list all of them though I might miss some. And I'll try to go from most mundane to most exotic.

A. Nothing. -- No pregnancy is possible due to their opposing natures. The two are entirely sterile or are at least partly sterile to one another. A pregnancy might occur but the child would most likely be still born or have severe complications due to its heritage. This might depend on how diluted the fiendish and angelic blood is, and the balance of law and chaos in the pairing.

B. "Human" Mix with no planar heritage -- The child is born with relatively minor complications if any, but the conflicting upper planar heritages have canceled each other out leaving the child a mix of whatever humanoid species went into the ancestry. Also the child might come out law or chaos biased if both parents descended from those sides of the axis. The same would probably be true if two tiefers, one descended from demons and the other from devils, mated. Keep in mind the result might not be anything like someone descended from chaos or lawful exemplars, but instead would be a mix of both sides' traits that show their chaotic or lawful heritage.

C. One side dominates -- Taking a page from genetics (the high-school version) keeping in mind that only half of each parents traits are transmitted it is simply a matter of which side comes out on top. The child might be tiefling or aasimar with the probability of which being either determined entirely at random or if one is descended from a higher form of exemplar than the other then those traits are dominant. Another way might be to look at the actual alignments of both of the parents. If they are both evil then the child might be far more likely to come out a tiefling.

D. A Weaker Mix -- Traits from both sides appear in the child but some cancellation has still occurred. This means that the child has fewer powers possibly keeping more of the penalties. Otherwise the child is perfectly viable.

E. An equal mix of both sides -- The child displays exemplar traits from both parents and sides of the axis but in the end is about as strong as every other child with that amount of exemplar blood.

F. A Stronger Hybrid -- The "negative" traits of both sides are fixed by the other and in the end it makes a stronger hybrid of the two. Some traits might have canceled each other but in the end the kid displays the better sides of both heritages.

G. Chaond/Tuladhara/Zenythris -- The good and evil aspects of the child's heritage cancel out and the child is born as if they were a descendent of whatever exemplar/humanoid blood mix is appropriate.

Tiefers (Aasimar and Tiefling Mix)
Due to the abundance of tieflings around Sigil and over the planes most mixes are either called by what they resemble the most (Aasimar if they look like an aasimar, chaond if they are chaotic ect.) or they are simply lumped in with other tieflings and just called tiefers for a generic upper planar mix. The racial traits listed below are merely for the most common found and are by no means representative of all or even half of the planar mixes out there.

Tiefer Racial Traits
• +2 Dexterity, +2 Intelligence, +2 Wisdom, +2 Charisma, -4 Constitution. Many tiefers proudly show off their uncanny charm, cunning, grace and wits but also suffer from the warring parts of their own heritage causing them to often be frail creatures. Their generally poor constitutions are also what probably keeps their numbers down across the planes as they are more prone to illness.
• Medium-sized humanoid
• Tiefer base speed is 30 feet.
• Acid, cold, fire and electricity resistance 5.
• Darkvision up to 60 feet.
• +2 racial bonus to bluff, hide, listen and spot checks.
• Automatic Languages: Planar Trade, home region. Bonus Languages: Any (except secret languages, such as Druidic).
• Plane of Origin: Any (Most often the outlands or Sigil).
• Favored Class: Paladin or Rogue (The first taken is considered the favored class and cannot be changed afterwards.)
• Level Adjustment +1: Tiefers are slightly more powerful and gain levels more slowly than most other races.