I am not sure if this is something that is widely known, but I will mention it either way. Off the top of my head, I know it works with the Midsummer Festival Tabard, and the Trophy Tabard of Illidari. There is a tabard vendor in both Org & SW. If you happen to destroy either tabard, you can go talk to the tabard vendor and he will give you a new one. If you take either tabard and put it in your Void Bank, then it will also allow you to choose another. You can pick either color, doesnt matter. Since the vendor is made to recognize your bags and bank, it registers that you have either lost or destroyed your tabard, which allows you to essentially have both colors in your possession.