The advantage of one handed weapons is versatility. You can use a single weapon set to have weapons against both generic and armoured enemies by equipping an autopistol and a plasma pistol for example.

One handed weapons provide two weapon skills each and those skills will occupy skill slots on the Skill Bar based on which weapon is equipped in which hand.The first two slots are dedicated to the right hand, the third and fourth to the left.

The only exception to this is when you have the same type of weapon in both hands. This setup will provide two additional skills that aren’t available when you just have one of those weapons. Two autopistols for example give access to the Crippling Salvo and Assault Gunnery skills, when one only provides the Short Burst and Full Auto skills.