Flex PCB Stiffener Capabilities

In a 2-layer Flex-PCB design, which stiffener combination can you produce?

a)      Side 1: Stiffener PI, Side 2: Stiffener PI

b)      Side 1: Stiffener PI, Side 2: Stiffener Steel

c)      Side 1: Stiffener PI, Side 2: Stiffener FR4