2:1 Herringbone Rome Gear

Oh yeah, that's right. Herringbone gears.

These are hard to machine with conventional tools, but on a 3D printer they should pop right out with minimal fuss. Wikipedia claims they're quieter and have a number of other advantages over conventional spur gears.

If my numbers are right, two of these should mesh with each other in a 20-tooth-to-20-tooth configuration AND a 10-tooth-to-20-tooth configuration...