I have actually be using a reef octopus bio pellet reactor to run both carbon and gfo
at the same time,...brs has a dual cylinder setup that runs both media in a separate cylinder using a very small pump to run water thru both of them ,.. I used this for a while but really didn't think this setup operated as efficiently as advertised so that's when I decided to use my bio pellet reactor instead,.....it's been running flawlessly for about a year and a half now and by using a larger pump I can easily dial in the right flow rate that is a nice compromise between the two different media.
You might have to experiment with what carbon / gfo
works best in your particular setup but for me I really like running " my" setup this way