[mike31154]

Best thing is to use one or both to simply make pure water for your system and/or drinking water. I'm not a fan of hooking these directly to a tank/sump for top off since intermittent, short runs are not an optimum way to run an RO filter. Best to run them for larger batches continuously. Whenever I start my system it takes a good 3 to 5 minutes before the RO output TDS gets down to 1 from a 210 TDS tapwater source. If this were to run for a minute or two intermittently 24/7, I would likely never get 1 TDS water into any top off system. Some things are better done manually, like filling a top off container or preparing pure water for mixing salt. Looks like you've got enough parts & pieces there to put together a killer dual membrane system if you wish.