Double Augmenting is a method of summoning a Cross Monster using Two Augment monsters instead of usually 1 Augment Monster. This method is similar to Double Tuning, except that you must used two Augment Monsters in your hand rather than on the field. In addition, you must first multiply the two Augment Monsters by their Levels before you can multiply the non-Augment Monsters afterwards.
For example, Red-Eyes Black Flare Cosmic Dragon needed a Cross Product of 49 to be summoned, meaning you can used a Level 7 Augment Monster, a Level 1 Augment Monster from your hand, and a Level 7 "Red-Eyes Black Flare Dragon" to Cross Summon it.
Downside of this method is that you would sometimes have a hand disadvantage, but it was far powerful than the usual method of Cross Summon.