A bifurcated web was a spell woven in upon itself, and the only known use of such a web was around the Palace of the Prophets which was used to slow aging.
The art of bifurcation was used by the wizards of long ago who combined both Additive and Subtractive magic to create two distinct shields. The first shield, or the outer perimeter, created an environment in which the inner shield (and its inhabitants) could exist in the world of life. The inner shield then was created (again using both parts of the gift) to attach the subject matter to "other worlds" giving the inner shield its special abilities, among them the ability to extend life. Essentially the bifurcated web would have two main power nodes - the first on the outer shield and another in the inner. The inner shield would be the most vulnerable point in the entire web should one seek to disrupt or undo it.