Simul-cast

Certain spells, such as Barkskin, can be cast in multiple layers at once. This is referred to in spell effect listings as "simul-casting." The entire grouping of spell layers counts as a single spell, and the entire lot is removed at once by anything that removes the spell in general. For example: simul-casting Barkskin twice upon a player gives two points of armor in each hit location and still only counts as a single Enchantment; however, one Dispel Magic used upon this player will remove all of the Barkskin at once.
Simul-casting still requires that the entire incantation for the spell be performed once for each layer. Simul-casting must take place all at one time; attempting to add another layer of an Enchantment with a lapse between the two wastes the Enchantment being applied, unless Stack or Attuned are in effect and a third Enchantment is not already in place.

Spells that may be simul-cast

Barkskin (may be simul-cast to two layers)
Bladesharp/Bludgeon (up to four layers)
Magical Projectile (up to four layers)
Protect (up to two layers)
Stoneskin (up to two layers)