The problem with one rod is that the craft might spin and the wing drag along the truss potentially causing damage. Another possibility is that during the ascent the wing remains in contact with the truss and freezes to it. Or if you really want to consider other possiblities there is nothing to say that the rod won't warp towards the truss and have the craft launch into it. This is still a possibility with a two rod solution however two rods require a shorter length than for one rod and so any warping will be less.
Furthermore I assume that the materials for the rods will be tested to ensure they don't warp too much at low temperature or if they did they would still be flexible enough that upon launch the guides would pull them back parallel.
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