I believe for the 1999 + Super Duty C&C DRW , dual diesel tanks have always been available: all WBs, all cab configs, XL and XLT. Not Lariet. At one point for the Lariets there was a conflict for space on the dash: tank selector switch vs a switch for some feature included in the Lariet trim. So for that stupid reason duals weren't avail in Lariet trim. Don't know the current reason.
Never dual petrol tanks for this series. I think the issue with dual petrol in the C&C is with the EVAP canister placement. Mid ship tank has EVAP can located where the aft of axle would be. Aft of axle tank has EVAP can located where the mid ship tank would be.
I'm guessing as part of the fuel system the can is required to be between the frame rails (although the diesel DEF tank is not always) or Ford has never had certified, can't fit in, or doesn't want a suitably sized EVAP can for dual petrol tanks.
And never dual tanks with SRW for this series C&C.
Dual petrol tanks on the Ford PUs are long gone, but not forgotten. I had a '91 RC with two small tanks.