There are no rules for bards that are being used until after you have finished dual classing. There's no difference between your dual class fighter/thief who wants to EVENTUALLY be a bard, and my dual class fighter/thief who has no intention of being a bard.
You have to be a dual class fighter/thief to be a bard, but that doesn't reach back in time to alter the dual class rules. It just means that the player is planning ahead to become a Brad, er Bard. At the point where the player wants to become a bard, he needs to check to see if he has qualified to do so, which means thief of the proper level range and fighter of the proper level range, among the other requirements.