Think about this - for the sake of discussion, let's say you have a 4000lb winch, and a snatch block, you double the cable, so you have an 8000lb pull - what's the load on the cable?
8000lb pull, doubled cable, so each of the two cables will see a 4000lb load - the only thing that's going to see an 8000lb load is whatever you used to attach the snatch block to the anchor point, and the anchor point itself.
Match the cable's working load rating to the winch capacity and you'll be fine.