Brew the mint tea with 1/4 liter of water and the black tea with 1/2 liter of water. Let the two varieties steep separately from each other, then pour them together and sweeten with rock candy as required (sugar or sweetener are probably fine, but rock candy is definitely better).
Makes a delicious tea with an oriental flair.
Note: You have to keep the ratio ½ to ¼, otherwise the tea will not taste that great!