Hm, I put both in a make shift soda bottle 'reactor', and now I have an 'official' phosban reactor I will put both granular carbon and GFO/phosphate remover.
Before both of those i put them both in the same media bag/filter sock.
Never saw what would be wrong with it. Didn't see any negative reactions from both being in same container.
Maybe using chemi-pure instead of carbor. This would also help minimize maintenance, because chemipure last much longer and phosban lasts a longer than carbon before it needs changing
Think of carbon like air in a protein skimmer, the longer the reaction time with water, the more effective it is. Same goes for carbon.
There is no specific flow rate that makes carbon suddenly start working. It is working no matter what. Hence the reason to keep all your jars tightly sealed, or it will start pulling contaminants out of the air.
I run both Carbon and Rowaphos in the same GEO 412 reactor.