Hi,
I just bought a 2012 RS Twingo and was confused initially when looking too.
I believe that the bench seats only came on the older Cup specific models (2008-2010ish I think), I think the CUP one also didn't have a few other things to save weight. But you could get a standard RS and then get the cup upgrade which added the wheels and suspension changes but kept all the other bits including the split seats. I think the facelifted one in 2011 scrapped the cup specific model and made it so the cup upgrade was an optional extra to get the cup extras and so you also have the split seats.
I just bought a 2012 RS which has the cup upgrade, I have the split seats. It definitely makes the ride firmer but supposedly sharpens the handling a bit too.
Happy Hunting!