This conversation came up quite a while ago, on a different post. Here was Var's answer to it.
"Affinity to elemental powers is directly tied to the region you belong to as the elements are the embodiment of the connection between individuals and their respective elemental god.
For instance, a Mangain civilian will have a stronger control of fire if he is in the region of Mangai, as that is where the element thrives. However if he leaves the region, his connection to Ikir lessons, and so does his control over his fire powers. The farther away he travels from Mangai, the less power he will have.
With this in mind, you can have mixed elemental babies in our world. However, the element that they will have control over will depend on which region they are in when they are conceived.
If a Mangain and a Nahoin have a child, and that child is made in the region of Naho, then that offspring will be able to control water. If the two parents decide they want to live together as a family unit, then the Mangain could choose to live with his Nahoin family in Naho at the cost of a reduction in his elemental powers.
Being born with more than one element is not possible."