Brian, as stated above, the two medias need to be run with different flows. It's very important to run the Phosban or equivalent GFO (Granular Ferric Oxide) at a low flow. The only movement you want from that is the very top layer of media to slightly tumble.
The carbon on the other hand requires a much more aggressive flow. Having it suspended in the reactor and tumble is the appropriate way to go.
You will hear some people say they run both the carbon and the GFO in the same chamber with success. I don't doubt their claim. However, if the GFO breaks down and enters your display tank, it can have disastrous effects on your coral.
I run both, in separate reactors with separate pumps.
Russ