Anyone notice this trend where the single disc dvd has the original artwork from the movie poster while the 2 disc edition has inferior artwork?

Examples are Iron Man, Dark Knight, Step Brothers, and Indiana Jones and the Crystal Skull.

Can anyone explain why the studios are doing this?

I would think it would be the other way around where the more expensive version of the dvd would have the better artwork.

yes that would make more sense!
step brothers 2disc is ugly and the biggest crime ever is the hancock 2disc with the movie spoiler!
i wouldn`t agree with you on batman and indiana jones though, and especially the iron man 2disc slipsleeve with the cut out section kicks ass!
however, most of the time the movie poster is the most classic looking and i usually prefer it over some crappy intern photoshop experiment any day.

I've always thought the cheapie single disc artwork for Moulin Rouge! to be much more elegant than the somewhat tackier 2-disc SE artwork.

I like the single disc cover artwork for Dreamgirls way better than the 2 disc. I have both btw! lol

Iron Man's 2-Disc art-is badass
The art you're most likely seeing is the slip-cover. Underneath is a hololike cover of Stark with the arc-reactor; there's a cutout on the slip-cover so it shows through.

I also believe Dark Knight's 2-Disc cover is actually the slipcase and the actual case art has The Joker.

I always thought because you are paying quite a bit more for the 2-disk set they should have both artworks in the DVD so you could choose which you wanted. The information on the 1-disk would have to be modified to accompany the true 2-disk set, but this way the people who pay more get a little more.