According to common sense, each kind of heaven flame acknowledged only one master. It was rare that a warrior could have two different types of heaven flame.
The warrior who wanted to fuse with a heaven flame had the restriction in his soul and body. Once he could fuse with the heaven flame, his power would skyrocket massively. Moreover, he could gain the special features of the heaven flame. However, it was difficult for heaven flames to fuse with each other.
The Ice Cold Flame was an icy cold, chilling to the bone flame, while the Earth Flame was a blazing flame. They were totally opposite in nature. It was almost impossible to combine them, as it was naturally repelling.
That was why the four were frightened on knowing Shi Yan also had the Earth Flame on his body.
The one who could keep two opposite heaven flames on his body should be an earth-shaking genius. Otherwise, it was impossible.