I'm assuming the combination of two chakra elements does not guarantee the same new element. Like w/ Yamato, wood is the result of his mixing water and earth. But wood is due to his particular chakra manipulation of the respective elements making for wood. The possibility exists that the combination of water and earth chakras could make for a new plant "element" or jutsu (probably the more appropriate word). The plant jutsu being the result of a different (dual) chakra manipulation from however Yamato manipulates the water and earth elemental chakras..