Any regular bi-pyramid would be technically fair. I think it's just that you tend to get rounder dice by using platonic solids or tesselating / modifying platonic solids.
I know the “Zocchihedron”, one of the first and best known commercial ones, is not fair (after this was proven, they were renumbered so that the over-/under-weighted results were better distributed over the range, but it's still not fair.)
> Most players use d10 pairs with a tens die, though, because they roll better.
There's also a nifty trick used in many RPGs based on d100 to roll once and have 2 d100 numbers (by inverting the result).
So for example in Warhammer you roll d100 for attack, you hit if your result is lower than your Melee Combat attribute, and the inverted result shows which part of the enemy body you hit (that influences critical hits and armor).
By the way, you can get d100 dice.