The problem with doubling your recipe is that the cook time does not double along with the ingredients. However, you will need to cook longer than what the recipe calls for. For example, if your recipe calls for a one minute hard boil two minutes for a double batch would be too long. A good rule of thumb is to increase the cook time by 50%. So in this example you would increase the cook time 1 minute 30 seconds. This is not a guarantee it will work, but will give you a much better chance of success.