In Cubase SX 3 you used to be able to get stereo rewire channels for Reason by alt-clicking in the rewire panel. In C4 they broke it. Read about it here:
A year later this is still not fixed. Read Steinberg's reply in this thread:
The reply states that they probably won't even bother looking at a fix until a 64-bit version of Rewire is released.
Dear very nice developers at Propellerhead HQ, if I send you some delicious ice-cream is there any way you could have a word with the guys in Hamburg to get this fixed? It seems that they wont listen to their customers so maybe you could influence things?