The static dissipative layer in a dual layer mat is still conductive, just not as conductive as the bottom layer. Conductive mat material is generally in the range of 105 ohms or less while dissipative is 105 to 1011 ohms. Higher resistances are insulative mats and will not disperse static charges, and they are not suitable for general electronics use.
A mat that is too conductive will conduct static charges too quickly, damaging components that may be charged before contact with the mat, so you are unlikely to see a benchtop mat with low resistance - unless you flip a two layer mat upside down, anyway. The idea is to gently remove charge.