i have never used twin di vessels, i have always thought it as a false economy, i have a tds at the moment of 40ppm usualy around 30ppm a bag last me around 6 months or longer, and cheap as chips, it works out pennys for me, the key is to run the di very slowly, i mean slow, if you use it at full tap water pressure, of course it will drain the resin fast, only fill the vessel to the shoulder, not filled to the top, which many have done, the bigger the vessel the longer your resin will last, not 2 of them linked,