Phantom & Batman fanfiction: Demon Twins, time warp, Chronically ill Danny, Commit to the bit

Fanfic where Damian and Danny are twins, but nobody besides Damian knows about Danny. Danny creates the illusion for the Bat-Fam that they experience a Time Warp, where if they acknowledge he didn't exist before the warp, he'll vanish. Damien and Danny play off his cold body and slow heartbeat as signs of a chronic illness; he also carries an oxygen tank and mask, which is why he never became a vigilante. He convinces Jason to take him to the museum, pretending it's a regular thing they do together—going to the reenactment section and taking photos. He also makes Tim believe they have a recurring sleepover where they play classic teenage sleepover crush games.