20/20

20/20 is a passive item.

Effect

Unlike other multiple shot items like Mutant Spider or The Inner Eye, 20/20 does not reduce the rate of fire.
The maximum amount of tears per volley that can be reached by combining and/or duplicating 20/20, The Inner Eye, Mutant Spider and The Wiz is 16, or 20 with the Keeper.

Blood Clot / Chemical Peel: Every second row of tears has increased damage.
Brimstone: Two Brimstone shots are fired, without an increase in charge time.
Diplopia: Every additional 20/20 adds 2 more tears in a tight pattern.
Doctor's Remote / Epic Fetus: Replaces the single missile strike with a barrage of two consecutive missiles.
The Inner Eye / Mutant Spider: Adds two/three tears to each shot, with no extra fire rate decrease.
Keeper: Fires 5 tears with no fire rate decrease.
The Ludovico Technique: The Ludovico tear gains one smaller tear orbiting the main one, each dealing the same amount of damage.
Monstro's Lung: Adds two tears to the tear barrage, for a total of 16.
Sprinkler: Fires two tears at once that almost completely overlap each other.
Technology: Fires two technology shots simultaneously.
Tractor Beam: Both tears are fired along the beam. Only one tear can be seen, as they overlap each other.
The Wiz: Two tears are fired in each direction.

Brimstone: Overrides 20/20.
The Inner Eye: Overrides 20/20.
Kidney Stone: Increase number of tears in tear barrage; however, only fires one stone.
Loki's Horns: Only occasionally fires one tear in the other three directions.
Mom's Eye: Only occasionally fires one tear out of the back of Isaac's head.
Mom's Knife: Overrides 20/20.
Monstro's Lung: Overrides 20/20.
Mutant Spider: Overrides 20/20.
Sad Bombs: Does not affect the number of tears when bombs explode.
Technology: Overrides 20/20.
Varicose Veins: No effect on number of tears when taking damage.