You can do this via policy routing.
Network Setup -> Routes -> Policy Routes
Specify the source addresses of the public DMZ, and select the appropiate gateway.
Excellent - works a dream - policy route
Forward Any DMZ-16 Any Any Port-B
Forward Any DMZ-8 Any Any Port-B
Where port-b is company A's DSL and DMZ-16 and DMZ-8 are address groups /28 and /29 respectively for the two DMZ ranges.