Jar'Kai

Jar'Kai was originally a style of swordplay that utilised dual blades, developed by the Yovshin Swordsmen. A similar dual-blade technique referred to as Niman was developed by the Royale Macheteros of the Kashi Mer monarchy. Niman was adopted by the darksider group known as the Legions of Lettow during the First Great Schism, as well as the Jedi Order itself.  Niman was developed into the sixth form of Lightsaber combat, retaining its name, while Jar'Kai became something of a blanket term for dual-blade combat in general.
It should be noted that the fourth form of lightsaber combat, Ataru, is also known to employ dual blades as part of its training regimen.


History
The Jar'Kai style, developed by the Yovshin Swordsmen, was named for a city on Atrisia. The city of Jar'Kai was also the site where the original Jar'Kai duelling sabers were crafted. These weapons were utilised by the Yovshin and were designed for the express purpose of being dual-wielded.
Niman, the dual-blade style developed by the Royale Macheteros of the Kashi Mer, was named for the dual triumvirate of the Kashi deities. The style was adopted by the Legions of Lettow, a group of Dark Siders responsible for the First Great Schism of the Jedi Order. After the defeat of the Legion at the hands of the Jedi, Niman was adopted by the Jedi order, eventually being refined into the sixth form of lightsaber combat, referred to by the same name. It should be noted that there were Jedi who engaged in dual-blade combat prior to the adoption of Niman by the Order; an example being Awdrysta Pina, who wielded a pair of Jedi katanas in battle. Many Jedi Knights and Sith Lords trained to use Form VI in the hopes of gaining a basic knowledge of dual-bladed combat, but very few ever achieved complete mastery of Niman.
Both styles predated the invention and usage of lightsabers, but being designed to be practiced with swords, they were easily adapted. In addition, the word "Jar'Kai" has become a blanket term for dual-blade combat in general, leading to the birth of the term "Jar'Kai tactic", which is to simply employ two blades in combat, regardless of whether one has received training in any dual-blade fighting style.
Dual-blade practitioners of Niman included Jedi Master Kavar, who utilised a shoto as his secondary weapon, and the Jedi trainee turned Sith Lord Exar Kun, who took up an additional weapon on occasion.  There have been few dual-blade duellists that practiced classic Niman, though the Jar'Kai tactic saw fervent usage. Numerous Jedi and Sith were known to take up a secondary blade in combat, though few did so regularly.


Description
Dual-blade fighters, whether classical Niman duellists or individuals applying the Jar'Kai tactic, are able to maintain a strong offense, as the speed of attack which two blades allow can simply overwhelm most opponents. A typical manoeuvre is to continually attack with one blade while relying on the other for defensive coverage, ensuring that a duellist can keep up their guard even as they attack. However, it is common that both blades are used for offensive purposes, keeping up a continuous wave-front assault.
Another advantage of Jar'Kai is that the two blades make it easier to hold off multiple opponents, as one simply has more blades to parry with. Dual blades can also be used as a defensive or compensatory measure when engaging another opponent who also utilises multiple blades in combat.
Known Jar'Kai techniques and manoeuvres include the "Rising Whirlwind", which consists of a duellist swinging their blades about their body, creating a "whirlwind" of deadly attacks, and the "Twin Strike", in which both blades are swung in an arc towards the opponent in a deadly power attack.
Though a complete fighting system in and of itself, Jar'Kai is easily adapted to other fighting styles.

Weaknesses
Despite the advantages offered, Jar'Kai is not without its drawbacks. Firstly, as both hands hold individual lightsabers, two-handed blows are impossible, and the duellist is unable to put all his or her weight behind blocks, weakening the defensive capabilities. Thus, an enemy with a single hilt can simply assault a Jar'Kai practitioners defence with brute force, battering away until the dual-blade duellist's guard gives way. However, this weakness is hardly absolute, as some duellists possess the physical strength to block power attacks with only one arm.
Secondly, because of the gyroscopic effect inherent in lightsabers, dual blades are more difficult to control than a single blade. Due to this, special training is required to practice Jar'Kai, and without it, a secondary blade is more often a hindrance than an advantage. Fortunately, dual-blade training is quite standard amongst most Force-using organizations, though advanced training is somewhat rarer.
A third trait worthy of note is that Jar'Kai practitioners are often overly reliant on their weapons and blade work, leaving their abilities in other areas undeveloped.

Weapon Specifications
As both Niman and Jar'Kai were developed prior to the invention of lightsabers, they were designed with metal blades in mind. Jar'Kai even had a type of sword created that was specialised to the use of the dual-blade style - the Jar'Kai duelling saber. However, due to the sword-like nature of lightsabers, the styles were relatively easily adapted.
Traditional Niman dual-blade regiments are based around wielding a short lightsaber, the Shoto, as the off-hand weapon, as the smaller blade is easier to control.
However, since such specialised weapons are not required to utilise Jar'Kai, many dual-blade combatants simply wield two standard blades in combat. In fact, weapons other than lightsabers have even been wielded as secondaries, examples including electrostaffs, swords composed of a lightsaber-resistant material, or even simple knives. Also, the number of lightsabers wielded is not limited to two, depending on the duellist's anatomy.