Contra-Contra

Are you going to put a motor in it?  If it is a static scale model I wouldn't bother.  The mechanism will be a more involved project than all the rest of the model. In contests I have not seen operating features on a model influence the final results much.

Do you have a lathe?  Even with a lathe and commercial source gears it would be a big project, especially in that small size. If you insist on doing it, you might consider friction "gears" instead of toothed ones.  No one can eyeball rpm by eye accurately enough to detect any possible slippage. 

Hello!

I thought about it for a moment, and maybe mating modified LEGO gears to custom axles could cut it for you? You can get angled gears smaller than 15mm in diameter from LEGO.

Hope it helps and have a nice day

Paweł

No motor. Long ago I did Seafire 47 with a contraprop, with just a brass rod for one shaft and a brass tube for the other. No gears, but when you blow on it they do turn in the opposite directions. The idea here was to just spin the prop with a finger and see the engine rotate in the opposite direction.

There is a builder in Russia who made it work with an SSW D.IV. http://www.dishmodels.ru/gshow.htm?p=9420 Unfortunately the Siemens-Halske Sh.III engine does not have the prop rotate in the opposite direction from the engine. He has a couple of pictures and a diagram, but I didn't really understand it.

Anyway, I had pretty much given up on the idea and thought I'd give it one last shot. Thanks for the answers.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXDLD4

This kind of thing might be a good start. I can't easily design the thing in my head, but there's a lot of bevel gear differentials for R/C cars.

The major gear diameter of this one is 19mm.

That is a good start! Thanks for that.