Yes you can, I did it in the past. It would be advisable to run two reactors or one at a time.
Both Carbon and GFO have a limited amount of capacity. How long depends on the water quality. GFO can be used up as quickly as a day at extreme levels. Carbon can last a very long time,
(I asked how long the city used there's for in the Water Treatment plant.....and it was on the order of many years),
I have never heard carbon working better under pressure ie (packed tight, so I will look into that that.......in regards to RO/DI filters, we want to ensure as much gets taken out in one pass....so its packed tight.....which also means not all surfaces touch water......we can pass it through a reactor many times and get the full use of the material.
However, both work via interaction with the water and a nice "dancing" motion for both which ensures the maximum water contact with all the surface areas of the material is recommended. It is because of this, and that they differ in mass....they one will either be getting ground up and passed in to the water or just sitting there........