The twins were identified as 15 year-olds by United and will move from Fluminense possibly as early as January.
Rafael told fifa.com: "We might be going across in January, but we're not sure. We've already started to learn a little bit of English - and yes, we're really looking forward to it. We're also looking forward to playing alongside Cristiano Ronaldo. We met him - and he said it would be good for us to join the club."
Fabio also said: "Anderson is also playing for the club too, so it will be good to see him. There seem to be a lot of Brazilian players in England now - Lucas is at Liverpool, Geovanni and Elano are at Manchester City, so it will be good to see them when we are in Manchester. I'm sure they'll help us adjust to living in England. But we're going over there to experience it for ourselves and learn more about football too."