No real harm, but totally unnecessary for a 6% ABV. Concur with Cheaton, depending on which kit you use, you may run the risk of overhopping. If you haven't bought anything yet, I'd encourage you to use one can of Coopers and one can of unhopped malt extract.
On second thought, I'd encourage you to forget the Coopers, and use malt extract, hops, and yeast as you desire. But if you want to use the kits, go for it. Adding the second yeast packet will probably result in overpitching, but should be OK.