I would not run them in the same reactor. You do not want carbon tumbling but you want the GFO to tumble slightly. Tumbling the carbon creates dust that can go into the tank- not good.
I thought you could run carbon and gfo in the same reactor, you just run carbon in the bottom with the sponges on each end then gfo on top so it can tumble...
I did it on a tlf 150 and it worked great.