Just pick one. I used the RIT application But I was also a transfer student, so I didn't need an essay and I was getting a fee waiver for being in PTK, so the RIT app just made more sense. The common app is good if you are applying to a lot of different schools because it saves you time. I'm of the opinion, however, that if a school has their own app it is better to use it. My logic here is that the common app asks a lot of questions that the school may not care about, whereas their app only asks what they want to know. But who knows? I'm just guessing. Either way, you only need to submit one.