It can be done, and both can be run at the same flow, same container, ect. Just remember that the flow requirements set are due to size/shape of grain and its ability to absorb the materials in the water due to its shape, size, and their molecular abilities to absorb things in different flow conditions. That info along with the fact that GFO tends to tumble in the container due to smaller grain and break apart if put through too much tumbling, releasing dust into the tank and maybe leach the Po4 back in to the water.
so, yes you can put them both in the same container and under the same flow. Your getting more out of the products if put under their correct flow requirements, that beeing separate containers with different pumps running their correct flows. They are just not running at optimal useage, and can cause your GFO to last a week or two rather than a 30 day period you would get under optimal. Same with the carbon.