The 'duality' of the Force is one of those odd misconceptions that keeps floating around. Yes, such a thing can be perceived, but it's like detecting potatoes and cabbage in a Beef Stew: they're certainly present, but they don't speak to the totality. Indeed, you could remove both and still have a pretty decent Beef Stew. The presence of the potatoes may contrast heavily with the taste of the cabbage, and it seems like the two shouldn't really go together, and yet, placed in the context of the meal, they're actually pretty damn delicious.
So too is it with the Force: whatever aspects may exist, they do so as part of a larger whole, and each can be embraced separately or together (as shown by many Sith and Jedi post-NJO). Most of the other Force-user Factions don't adhere to Light or Dark: those tend to be the realms of the Sith and Jedi, who perceive the Force in a way that opposes one-another, failing to recognise that Light does not exist without the Dark, nor vice versa, and that the Force requires both, but more besides.
The conflict between the two factions is, ultimately, ideological: it comes down to their perception regarding rule. Jedi believe they should not rule, and thus spend their lives trying to maintain the 'democratic' system of government that they misguidedly believe will create the peace they so desperately want. The Sith believe they have the right to rule, and will seek to obtain power because, when the strongest rule, peace is a natural byproduct, for opposition is outgunned and heavily stomped on. The war between the two is political: the Sith seek power, the Jedi wish to stop them. That's all it comes down to.
And, let's be frank, very few of us have a personal stake in the war: we fight because we're taught to fight, told that it is natural to our factions to fight, and that our enemy opposes our way of life (which is true). It's natural to imagine that the two factions might be reconciled, or at least persuaded to stop using their powers to harm the Galaxy - fight each other individually, if you wish, but the conflict must cease bringing innocent lives into danger. The recent incident at Korriban is a perfectly good example: innocents dying because the two Factions can't keep their fighting confined to one-another.
It's nonsensical, and it has to stop.