"The condition is essentially the lack of contradiction between the two genjutsus, or more if a plethora are utilized. An example may work out here, let's say that a person first used an A-Rank genjutsu causing the opponent to feel a sensation of immense heat, but then cast a B-rank one causing a blizzard to go by, the problem with this is that there are no situations in which a blizzard can cause one to feel immense heat. This creates a contradiction within the chakra streams within your opponent, thus causing them to not compliment one another; henceforth, the two genjutsus are incompatible with one another. Therefore, the stronger genjutsu overwhelms, instead of compliments, the weaker one, and thus the opponent will only feel the heat."
"Now, let's slightly change this. The genjutsus are the same, but now both genjutsus are B-Ranks. What occurs here is that they still conflict with one another, but instead of the heat genjutsu overwhelming the blizzard genjutsu, both of the genjutsus cancel each other out. The reason behind this is that neither one can conquer over the other, and thus they both eliminate themselves, leaving the opponent free from your own genjutsus without putting in any effort towards eliminating them themselves. Thus, care is needed if one is to utilize genjutsu layering properly."