This is a great way to increase your brewing capacity without investing in new equipment. Just be sure to brew with 10 gallons worth of ingredients (minus the water which is to be used later to water down the wort). Even some breweries use this technique of high gravity brewing. If you were to increase a 10 gallon wort to 15 gallons, some say it is best to ferment the 10 gallon wort first, then add water down with boiled (de oxygenated) water befor carbonating. But when doubling your wort size, I would say go ahead and water down before fermentation. Just be aware that this procedure will not do you any favors in the flavor department, but the (slight) trade off for twice the amount of finished product definately has its advantages.