uninsured, underinsured auto insurance

Redoing my auto insurance through State Farm in PA. I am increasing my uninsured and underinsured coverage to $250,000 for each individual and $500,000 for each accident (I am doing this on both of the cars my wife and I own) and my agent advised me to andquot;stackandquot; the coverages. As I understand it that would essentially allow me to double each coverage ($500,000 individual and $1,000,000 for each accident) because I would be able to use the coverage from each car if I needed. The problem for me is the cost is so high and I already have what State Farm calls andquot;Qandquot; coverage for a million $ to pay for medical expenses and medically related costs in case of a debilitating accident, so I view this andquot;stackingandquot; as almost a double coverage I don't really need. I want to be covered in case of a debilitating accident (my wife does a good bit of driving for her job and we really depend on two salaries to pay for our living exspenses) but I feel this is overkill. Any advice would be greatly appreciated.