Ideally you want to use two different reactors here. The flow rate you want through carbon is different from the flowrate you want through GFO. With carbon you want a comparatively slow flow, to give the carbon a chance to absorb everything. With GFO yoiu want a faster flow rate, enough to just keep the media in suspension, similar to a fluidized bed filter.
As for plumbing you can use one or more T's off the main return pump and valves to control the flow rate to the tank and the media reactors. From the media reactor, you can plumb the water either back to the sump or to the display tank.