Dual Specular Lobe Indicates whether to add a second specular lobe to the base layer to simulate hazy materials without using a coat. If you also enable anisotropy, each lobe has an independent, configurable, anisotropic smoothness.
The two lobes that characterize the base surface layer use the same index of refraction, or specular color (depending on the Base Color Parametrization). The second lobe then further uses a different input configuration depending on the Dual Specular Lobe Parametrization.