The feat Spear Dancing Style says
While using this style, you grant the chosen weapon the double special weapon feature, using the weapon’s normal statistics for its main-hand end and the statistics of a light mace for its off-hand end.
(Emphasis mine.) This GM would rule that the spear's off-hand end remains a spear—and specifically still that spear that the wielder's using, even if that spear's a magic spear—, despite that spear's off-hand end possessing the statistics of a light mace. So in this GM's campaign, a warrior that possesses the feat could buy a lone +5 flaming shock spear and reap the benefits of that +5 flaming shock spear both when the warrior pokes a dude with it and when the warrior smacks a dude with it. (To be clear, that warrior's off-hand attack with that spear would use the statistics of a +5 flaming shock light mace.)
The warrior has expended valuable resources on what's widely considered a subpar fighting style with a simple weapon. This GM would view the warrior as having earned the right not to have to, for example, make magical separately that spear's off-hand virtual light mace!
The weapon modification dual balanced applies only "[w]hen wielding two weapons." Although the warrior that possesses the feat Spear Dancing Style is using the spear as a double weapon (there's no good direct link—search this page for double weapons), the warrior still wields just one spear, so there's no benefit to the warrior in that situation for the spear having the weapon modification dual balanced. The warrior must use two different, separate weapons to benefit from a weapon with weapon modification dual balanced.