Although both media are used ...

Although both media are used for water treatment, there is a reason these processes are separated.  The activated carbon is typically used a protection treatment step to remove oxidants which are deleterious to the resin.  In addition, the microfine particles electrostatically clinging to the GAC could wash off and adhere to the resin.  Once coated with the fine AC, the resin is essentially useless or at least, significantly diminished in capability.  Best to separate the processes rather than pay a premium price for a mixture that will be moderately effective at best.