I had a dual reactor. I actually still have one. It's awful. The loss of flow into the second reactor was awful. If you can't find anything, I'd just DIY it. My DIY one worked the best out of everything I have run.
As for GFO and carbon together, don't do it. They both require different tumbling speeds to be effective. There are products out there that are 'combos' like this, but aren't specifically a carbon/gfo combo to accomplish the same goals.
What are you using for media? Make sure you flush the carbon block if it's in front of tfc membrane
The DI needs replacing before most others ..Often 2-3 times before the others depending on the source water.. My unit shoots 10ppm with out the DI due to high TDS from tap [350+] There is a calculation that estimates most RO units remove as low as 90% of TDS..
The DI does the remainder..
The TFC should last up to 3 years if the other filters at maintained properly..
I run 5then 1 micron before my 2 carbon blocks..I change the 5 2x as much as the 1.
I change the carbon blocks every 6-9 months depending on how much I am using..
Replace the 5 often as it will prolong the life of all filters after it.
The TFC [Thin Film Composite] is the last filter before the Di in most setups and last on set ups without DI.
It is the filter usually in the non clear housing sitting horizontal on top of unit...
It is the most expensive of filters and should last 2-3 years if you care for the filters before it.
The Di expires the fastest of all media used..
I like the BRS color changing DI media as just one look tells you where it is at..It starts blue and dies yellow....
TDS will 'migrate' through the TFC when the unit is off..This is called osmosis as the water seeks equilibrium inside the unit without pressure...
My Tds is350 from source.When I open my ro the TDS meter [built in BRS] it goes up to 200 sometimes [quickly] and then returns to 10-20 in under 30 seconds..I have installed a T after the TDS sensor and discharge the first bit of water until it gets back to low TDS..
I use a lot of RO water without DI for my fresh water breeding efforts..I use the DI[have BRS dual DI] for my marine tank.It is always zero or I change one of the DI units..
If aquatic life is available I’d get a twist unit. Love mine.
if you have a turkey baster blow them off don't give it a chance to settle in again .
I like the skimmer /or pump for skimmer as close to first in sump as possible.
I use bio pellets .If you have had the first dose in reactor for a month add enough to have full dose.. On skimmer placement you should have the return for bio pellet reactor feed near skimmer intake as it needs to be skimmed to be complete.
Keep the light on 24/7 or opposite tanks light cycle where ever it is for the cheato ..