It really does not matter, a single good 0.6 micron 20,000 gallon Chlorine Guzzler carbob block is more than sufficient for both free chlorine and chloramines.

Many vendors try to scare buyers in to buying "special" chloramine filters. In truth, chloramine removal is not that tough. You need good DI resin much more than any special carbon since a good carbon block will adsorb the chlorine portion of chloramines, its the ammonia portion many overlook and thats where the quality DI system comes in to play. I would upgrade my DI system and drop back to a single 0.5 or 0.6 micron Chlorine Guizzler. Less pressure drop across the membrane so more production and potentially higher quality effluent.

Dual carbons are a holdover from years ago when carbon was not as refined or understood as it is today. On many of the ebay quality systems it is used to make up for low quality, high micron or unrated size, and low capacity carbon, often in granular form. Many granular carbons are exhausted in as little as 300 total gallons, thats 240 waste gallons and only 60 treated RO/DI gallons. Some carbon blocks only last 1000 to 1500 gallons so again thats only 200 to 300 treated gallons.

With the advent of carbon blocks such as the Matrikx+1 from KX they can now blend and extrude carbons for maximum performance and lifespan. get a single Chlorine Guzzler and convert that empty housing in to a second DI for better water. It will extend you DI life tremendously.