For "wear" parts (U- and CV-joints, bearings, tie rods, etc.), unless it's a odd or premature failure, I usually replace in pairs, or however many there are, figuring that the others can't be far from failure. For non-wear parts, I tend to repair as needed - the other side may never fail.
Also consider failure history of the gaskets and cost vs inconvenience of doing both at the same time or separately. In this case, it seems that the cost would be about the same for each side. If the engine had to be pulled, or similar, then I might consider getting them both done at the same time.
My gut says fix what's broken this time.
