In that case:
Lutowslaski's Paganini variations
Scriabin's Fantasy (short and not hard, and my favorite 2 piano piece I've played)
the Scaramouche as mentioned, can be used as good contrast as well since it's all tropical and stuff

Poulenc sonata for 2 pianos. (the first bars are the hardest to play together, but if you do, you'll impress a lot of people)
The Rach suite, it's a good option but depending on your skill and time you have to prepare yourself. The 2nd and 4th can take a while to get to tempo; and the 1st, if you're not used to Rach's style, can be tricky. You can also try the 1st suite in any case.
Or, if you want to go Hardcore, Bartok's sonata for 2 pianos and percussion.