It is normally recommended to keep them separate. The GFO is harder and if it rubs against the carbon it will grind it down and release carbon dust into your tank. Your GFO should also tumble in the flow and the carbon should be tightly packed. Next time you reload it, I would run the carbon between the sponges packed tight and put the GFO above the top sponge so it can tumble. Well, as long as the plastic end strainer will keep your GFO in. I know mine does. I run my GFO reactor without sponges.
Agreed on light tumble for GFO and pack the carbon.