Double Vis is a variant of the normal Vis added in the Wrath of the Lamb DLC.
They are similar to the regular Vis, but their skin is much more pale. They also have blood around their openings, and have another gash on their back.
They function identically to regular Vis, but they also fire a blood beam from their back.
Depending on the frame that the Double Vis attacks, one side of the blood beam it shoots will not deal damage. This means that if you are "hit" by a Double Vis's laser, you actually have a 50% chance that it will not deal damage.
Eternal Double Vis' behavior is mostly the same but their Brimstone beam will leave red creep.