I'm no battery expert, but it is my understanding that deep cycle batteries are designed to deliver moderate current draws for an extended period of time. They do not hold up well to high discharge rates. For your winch you might be better off with a normal automotive battery. They are designed to deliver high current to a starter motor.
In general it is not a good idea to connect 2 batteries in parallel. They will never be at exactly the same voltage, and you will get current flow from one battery in to the other.
Stan
In general it is not a good idea to connect 2 batteries in parallel. They will never be at exactly the same voltage, and you will get current flow from one battery in to the other.
Stan