Doubling up the Fire Spell

[Helborn]

Can a second fire spell be cast into the same or an overlapping area to double the damage caused?
I was playing the Castle Defiant siege scenario from Orcslayer and my mage wanted to cast two 7 hex Fire spells overlapping in front of the gate once the ram came close.
I contend that the hexes that overlapped had two individual fire spells and the total damage would be 4 to pass through and 8 if standing in place.