I am using GAC and Phosar together in a phosban reactor. When I started discussing this with Dr. Randy Holmes-Farley last year, he was using both in his canister filter. He thought that a canister filter forced water through them more efficiently without pulverizing them. Then he stopped using GFO because his macroalgae stopped growing. He and I agreed that his macroalgae was getting phosphate limited and no longer growing. I have had the same situation using GFO in the past, with or without the reactor. When macroalgae becomes phosphate limited (I even killed a batch of chaetomorpha), they stop taking up nitrate as well. (I see this phenomenon in various planted aquariums all the time.) As a result, my nitrate and his nitrate levels hovered around 0.5 ppm.
As for the ratio, he did not indicate any particular ratio. He said maybe 5% GFO to 95% GAC. I assume by weight and not by mass, but I am not sure. Phosban at full dose is supposed to last up to 6 months, but if you mix it with GAC, you will be throwing phoban away prematurely like after two weeks to a month. This is the reason that people use a much smaller amount than the recommended dosage by GFO manufacturers. It is a balancing act. If I use too much, my chaetomorpha dies. If I don't use them at all, I get a small patch of cyano when I use too much Golden Pearls.
I suppose that each person needs to try out what works for him or her since our tank conditions are all different. However, I would not worry about the precise dosage. I know that things are all right in general as long as my acros look bright and colorful and they are growing really fast.
Tomoko