Make this a split release. Call it Pokemon Major and Pokemon Minor. Major has all the Pokemon originally available in the warm color releases (red, gold, ruby, pearl, white, y) while Minor has the Pokemon in the other ones (blue, silver, sapphire, diamond, black, x).
A lot of Pokemon are available in both versions of each release - only a handful are really "version exclusives," so I don't how this suggestion would work.
Also, I don't see a big problem with two artists doing two pokemon from the same evolution line. Not everyone is going to have the same general approach to composition (stylistically speaking), and while Gastly and Gengar (for example) have the same typing and a lot of the same moves, they're still two different pokemon technically. One poisons people by suffocating them and the other drains the life force out of people by mimicking their shadows... (wow pokemon is pretty grim sometimes). The two compositions will be similar in some ways, but it's highly unlikely that they'll be exactly the same in my view. I spose it's down to Ateno to decide tho.
...(except we're not really. Pooor confagrigus 
)
Great work
.. more than Winter Chip, haha