According to the latest issue of Game Informer, The Beatles: Rock Band may offer support for multiple singers.
Destructoid reports that the May issue’s "Loose Talk" section proposes that the game will allow multiple singers to harmonize using different microphones. It seems very probable, considering that the pricey special edition will come with a microphone stand — and obviously one must consider The Beatles’ rich harmonies.
In addition, the Game Informer also hints at user-generated content for Rock Band in the near future. Though Harmonix has generally been the innovative leader in the rhythm game genre — pioneering the original Guitar Hero, as well as drums and singing for Rock Band — Neversoft beat them to the punch by creating ‘GH Tunes,’ Guitar Hero World Tour’s content creation system. It’s pretty believable that Harmonix would play catch-up here: Rock Band has to remain competitive, after all.