Will Carbide 300R fit 2x GTX 680 SLI

It depends on the particular card you want to use, as not all GTX 680s are the same size, for instance the EVGA GeForce GTX 680 SC Signature is 25.4cm long, while the ASUS GTX 680 DirectCU II GTX680-DC2T-2GD5 is 29.9cm long.

Unfortunately the only figure given for GPU clearance in the 300R that I've seen is 45cm (when it avoids the HDD cage), so I don't know the limit for the second card.