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In the books Geralt usually only carries one sword on his back the other one is mostly stored on roach, which one depends on the situation hes in. Additionally he can kill almost anything with his normal sword, the silver one often just makes it easier so he wouldnt be screwed if somthing surprises him, which rarely happens as monsters are more rare. The games changed that as the player wouldnt have that much to do and because you cant ask the player to always run back to the horse to switch swords.