I'm thinking about buying 2 Coopers extract kits and adding them together. Since each kit comes with a packet of yeast, should I just add both packets of yeast to the brew? And at what strength ABV do I need to look at high alcohol yeasts?
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.