Another Another issue with Kent is that it is Alumina/Aluminum Oxide (Al2O3) and releases aluminum which some corals, especially softies, which often do not respond well, although this is not always the case. Alumina is also not very good at removing PO4 compared to GFO, You need the PO4 levels below 0.50 ppm or you will be spending allot of money. Here is something on GFO's
Iron Oxide Hydroxide (GFO) Phosphate Binders by Randy Holmes-Farley - Reefkeeping.com
You may want to review this post if you have not seen it.
Is aluminum safe in saltwater?
On reactors the media tumbles in a tube, it is fluidized, where it should be at a low rate. If the tumble rate is to high the grains start to break up. Many people do mix the two, GFO and GAC. The best media for reactors or canisters and especially canisters is Warner Marine PhoSar. This is not a solid GFO like the other but is porous like GAC. It is much better at removing PO4 than any media.
One of the other issues about GAC and GFO are their length of time to be effective. In short GFO's last longer than GAC. So in short, with one reactor, where they are combined you may end up wasting GFO.
In canisters, with media bags, you do not have this issue but others. GFO like Phosban or other GFO's, other than Warner, are not suppose to be used in media bags in canisters, although many do. They work best in reactors. A solid media is much more subject to channeling, chimney effect and flow rate loss. Warner GFO does not restrict you as much in flow rate as GFO's like Phosban or others. What ever media you use, slow flow it better to a point be it PO4 removers or GAC. But as rick pointed out together you will not reach 100%.
2 cascade 1500 canister filters
Those are nice canisters for FOT or FOWLR but are overkill for a reef tank for your purpose, using them for media (way to big). You could use them though. For maximum efficiency, of a media in a canister filter, "positive flow" types are the best, of which yours are not. Eheim's like these below are positive flow.
