Dipper is Samey, because they are known by their nicknames. Dipper, because of his birthmark. Samey is pronounced as Same-Y, instead of her given name Sammy, because she is the inferior sister to Amy. If Dipper was the inferior twin, then he would have to convince people to call him Tyrone, because that's the name he was wanting.

Mabel as Amy, since they are the twins who have common birth names.

Auditions
Dipper introduces his audition tape as "Tyrone Pines". He even denies having a sister. Mabel overhears this and tells Dipper to get his butt downstairs. Dipper quickly ends the audition, telling...