Paragon Multiclassing is something different. If you take all the power swapping feats, you can forgo a paragon path and instead get more abilities from your other class.
As Chase said, if you take the first multiclass feat, you qualify as being part of that class. If you are a fighter and take a Warlock multiclass feat, you can take either Warlock or Fighter Paragon paths, or a path that requires both, or Paragon Multiclass. Keep in mind that not all multiclass feats give you all the features of a class. For example, the PHB multiclass feat, Pact Initiate [Multiclass Warlock], will qualify you for having a pact, but the multiclass feat found in Arcane Power Student of Malediction [Multiclass Warlock], qualifies you as having the Warlock Curse class feature.