I don't see why not have them as separate downloads. I doubt there are a lot of people who wouldn't want to listen to the audio otherwise but then, once they already have it on their computer, decide to go ahead with it. It's more likely that the choice they make about format will be influenced by what's presented in the post as the idea behind the challenge and so on, and perhaps a request you make that if people are on the fence, they give the podfic a try and experience the story the way it was intended to be or whatever.

I do think having the text as DL instead of straight in the post takes away the sense of "the default is to read, and since this is right in front of me I will go with the default." It makes it more of a measured decision which format you'll go with.

Also, as people say, if the text actually is written in a way that fits better with reading than writing, some people may choose text, read, and then be like, "you know, now that I'm seeing this, I actually think I'd really like to hear it acted, because it doesn't seem like I'm getting the full experience just seeing it." Perhaps.