In the Belgariad and the Mallorean series, there is a small order of sorcerers who basically made sure things worked so that the good guys won. The main two, Belgarath and Polgara, are followed the most closely as they are crucial to the events. Beldin, while not very present in the Belgariad, is crucial in the other series. The last two, Beltira and Belkira, never show up. What were they doing that they couldn't help out?
Their role isn't really detailed in either the main Belgariad or Mallorean series but is expanded upon in both prequel novels (Belgarath the Sorcerer and Polgara the Sorceress)
As @Mark Gardner says the twins were principally working with the two codexes of the light prophecy (the Mrin and Darine).
They were uniquely suited to this due to their close mental bond as twins allowing them to work both documents simultaneously and spot the correspondences between the two, and the way prophecy works in the Belgariad universe is such that the consciousness behind the prophecy (the "Necessity") only "allows" certain elements of the prophecy to be understood at certain times - often as the prophesied events are approaching. This means that the twins were best utilized at the Vale studying the codexes so that as and when world events allowed them to shake loose something from the prophecies they could discover it as quickly as possible and communicate the information to Belgarath et al who would then act on it out in the world.
As Belgarath puts it in his prequel novel:
"That pair of gentle Alorn shepherds have been so vital to what the rest of us have done that in a rather special way, they've been our guides. We run around the world in response to what they discover. The Necessity usually doesn't bother to talk to us. It talks to the twins instead." - Belgarath the Sorcerer
And as a side note they do show up, at least in the Belgariad they are there in the Vale when the party has a brief visit during the hunt for the Orb and they provide sorcery support to the armies of the west during the Battle of Thull Mardu