Yes, not a problem. You will need to slow down your flow in your reactor as compared to what you could do with carbon only. Also, you will waste some of the life of your gfo, as you should change it out every 4 weeks because of the carbon.
You can also run less GFO in the reactor so that when you change it out you are not wasting any. If you usually put 3/4 cup in for 6 weeks, instead put 1/2 cup in for 4 weeks.
I did this too but ran into difficulties. GFO is meant to lightly tumble and Carbon needs to be held in place tightly with no tumbling. I attempted to do this in a single BRS reactor setup and it just didn't work right. I tried to get the flow rate correct but I would always end up with the GFO not tumbling or the carbon sponges moving causing dust. I would highly recommend separate reactors so you can tune the flow correctly.
I have run reactor's off a single pump in the past, you will need to put a ball valve on the input side to dial each one in to it's optimum flow. I'm not sure a MJ1200 would be enough, I guess it would depend on the size of the reactors and the amount of media in each.
BRS has a video on how to run them both in the same reactor. You need a certain amount of carbon to keep the gfo from binding together, you also need a piece of foam to hold the gfo and carbon in place and to keep it from tumbling which can cause the carbon to break down.
I have an Atomic Reef reactor and separate it with their media sponges. I put the carbon in first and then the gfo. The carbon doesn't move at all and the gfo very lightly tumbles. The output goes into a filter sock, just in case any media escapes.
right now, I put Carbon in a filter sock where water returns (this may just works fine, but I don't know). I have a valve to control the reactor water flow, but I would like to open it up a bit more so I was thinking adding Carbon in GFO reactor.
Bump. I had this question too, as I have been doing it for a couple of months. Glad to read that others have success as well. I put them in with GFO on bottom with a spacing sponge, then, it sorted itself out the opposite. The carbon is on bottom and the GFO tumbles lightly on top.
I ran mine together for over a year.. Mind the two together and put a sponge (well 2) to keep them from moving.. GFO only needs to tumble so it doesn't stick together. If you mix it well enough it's not really an issue. Now I user BRS GFO other brands might be different.