The theory with some kits is with only one upper or really a specific replacement single upper, it reduces the inherent binding that the stock uppers can cause so your rear stays centered AND does not bind up during movement. All kits are engineered a certain way for a reason with either none, one or both or a complete replacement. I'd stick to what the your specific kit suggests.True, you still use the uppers. Some guys say that you can take ONE upper out. I'd left both mine in.