The Twins (Retinazer and Spasmatism) are the Hard mode version of the Eye of Cthulu. They are giant mechanical eyes that are linked together (although they can separate to fight in different areas). They can be summoned anywhere by using a Mechanical Eye.
NOTE: The Mechanical Bosses can only be summoned after dusk (7:30PM) and they must be destroyed before dawn (4:30 AM). It is also wise to spend the time making an Arena as a place to fight the Hard Mode Bosses, especially if you going after them solo. You will also want to utilize potions and buffs, such as Ironskin, regeneration, Thorns, etc. to make the battle easier on you.
RECIPE - make at an Orichalum/Mythril anvil
7 Souls of Light
5 Copper Bars
5 Iron Bars
1. Focus on destroying one of the eyes and try to keep the other above half health.
2. A weapon that shoots or has long range is ideal since they attack from a distance. Vampire Knives are your best option, or a rapid firing gun.
3. Wear sturdy armor (Turtle preferred) since Retinazer shoots lasers and Spasmatism shoots a cursed flamethrower, so you will get hit a lot. Also might be wise to wear a Cobalt/Ahnk Shield.
***Same as 360 version***