From The Inheritance Encyclopedia

Blagden's riddle is told to Eragon by Blagden in Eldest when he is in Ellesméra. He recites to Eragon:

While two may share two, and one of two is certainly one, one might be two.

Christopher Paloni releaved in Brisingr that Brom is Eragon's father. This riddle is believed to have foreshadowed this fact.

[edit] Meaning

"While two may share two," means it may seem that Murtagh and Eragon share two parents.

"And one of two is certainly one," means one of the parents is certainly the parent of both.

"One might be two," means what may seem to be the other parent is actually two different parents.