I've just Revived (kind of successfully) the Trio-Species challenge. Now I'm going to revive a very, very similar challenge -- the DuoSpecies challenge!
First off, I don't remember all the rules, but the basic one is:
YOU MUST COMPLETE THE GAME WITH 2 POKEMON.
That's pretty much it. You get to pick the two Pokemon that you want, and then you progress through the game using only these two Pokemon.
Here's the sign up sheet:
And here's my sign up: