I think the biggest issues with mixing them are (1) carbon and GFO usually have a different flow rate and (2) I usually swap out carbon more frequently that I do my GFO. However, for the sake of convenience, it's not a bad idea. Just know that carbon gets used up before the GFO does and make sure the carbon doesn't tumble in to dust. GFO has a tendency to clump and when that happens, you'll get little streams of water going around the clumps. The clumped areas don't get any water flow and don't really do anything. However, if you have enough flow so that the GFO is almost all suspended (light tumbling particles on top), the carbon might tumble too and it'll get smashed up against the harder GFO particles.