Jar'Kai was a blanket term for dual wielding taken from a much earlier form of Sword combat, Niman was also an ancient sword-style not created by the Jedi. Much later in history, when the Jedi devised their sixth form of lightsaber combat, they named it Niman but also taught the history of its origins.
Niman isn't necessarily a dual weapon style, only that it lends itself well to said style. Ataru can also utilize two blades within its structure.
I'm assuming it has two names because of some slip up in continuity in times gone by lol. From what I can decipher, Jar'kai was created during the early nineties Expanded Universe, The Jedi Knight series I think. Then during the prequels I assume Anakin's style was called Niman in a visual guide or something... happens all the time. (Could be wrong, just a guess)