Maybe. Llama will let us know when he starts, and there's no problem with downloading both. When our turn files arrive, it'll automatically activate the mods for us.
I've seen that error message before - but not when 2 slots are open. I've only seen it when 8 out of 8 are submitted, or something similar. Sorry I can't be more helpful- but I recommend PMing Llamabeast. It's out of my hands until the game is up and running. Then I can admin things.