Crossfire only needs one, the two connections are parallel to each other. In fact, using a second used to increase the probability of errors and "snow."
Only use a second if symmetry is really important to you when you take pictures of the set up
or you figure you will lose the second if it is not in the system.
Whether you use one or two is like asking if you should put your pants on before your socks. Some people may care one way or another.. but really.. meh.