I run a dual reactor, the carbon side runs down through the media so it doesnt tumble and grind up the carbon. The GFO side runs up through so the media tumbles, as GFO should or it clumps up.
I used to run the BRS mini with carbon and GFO mixed. Your running it right AMILLER400. You should not tumble the media when running them mixed together as the carbon is harder and will grind the GFO to dust. Also running them mixed together, the carbon keeps the GFO from clumping.
As to whether you NEED to run carbon in a reactor, use carbon at all......
BRS did a nice video of the difference between passive use(bag in the sump), and reactor use. The difference will surprise you.
I'm old school and have always used carbon from way back, so it just feels wrong to me if I don't, but a lot of people don't and their tanks are just fine too. I would say if you have softies that participate in chemical warfare, it's necessary, otherwise it's your choice.