Arcane Stretch

When cast twice in successive actions, this spell temporarily creates 1 spell slot one level above the caster's normal limit.  If concentration is lost between the two casts, the spell fails and the spell slot is lost.  The spell slot is retained until used or concentration is broken.  The caster must know the higher level spell to cast it.  At higher levels, successive casting at the higher level creates a spell slot of the next higher level.