There is in fact a style of fighting which utilizes two spears. The Irish used to use single-handed spear combat. This style of fighting was used only by those who already had great skill in double-handed spear combat due to the requirement for greater control and strength.
In this style of fighting, one would carry a one-handed spear in their strong hand, and either a large shield or a small shield and spare spear in their right. While the combat style wasn't specifically designed for fighting with both spears at once, this is clearly where the inspiration for the movements of the character are derived. Some of the ways he strikes in the show even have great visual resemblance to this fighting style.
Add that to the fact that his character is of Irish legend, one of the most famous knights of Fiana, and that within those legends, he does have two spears among his collection of mythical weapons, it leaves little doubt that our Irish, spear wielding combatant would be using Irish single-handed spear combat.