Alternate-Universe Squidward appears instead of normal Squidward, during the entirety of the scenes in Plankton's alternate universe.
"Squidward in Clarinetland"
During Squidward's insane quest to get his clarinet back, two Alternate-Universe Squidward's appear in the mirrors on his left and right and torture him by stuffing his clarinet in their mouth's and playing them loudly.
Mirror Squidward appears several times during the episode. He appears whenever Squidward looks into something reflective. AU Squidward is the person Squidward takes to the Galleria Diphtheria as Squidward made SpongeBob carry a mirror. In the end, Alternate-Universe Squidward tells the real Squidward that he will always be his plus one, which delights the latter.