Olivia and Stewie
From the animated television series Family Guy (01/24/2002). In the "From Method to Madness" episode, precocious infant Stewie is put in the Quahog School of Performing Arts. Simon, his teacher, pairs him with similiarly precocious infant Olivia, who's as arrogant as Stewie. Their final project, a duet, wows Simon, who puts them on a local tour singing "You Do." Their opening act at the local theater is the unlikely trio of Garfunkel, Oates and Nash. Stewie's and Olivia's enormous egos quickly clash and Stewie tries to go solo, but flops. Olivia gets a movie role as Stewie descends into madness.
Stewie voiced by Seth MacFarlane, Olivia by Seth's sister Rachael MacFarlane