A dual chemical feeder device treats a fluid simultaneously with two different chemical compositions which may not be wholly compatible. In an exemplary embodiment for application in swimming pools, hot tubs, spas and the like, a chlorinator is combined with a sanitizing canister in a chlorinator/sanitizer to chemically treat the full flow of water through the apparatus with a chlorine-based chemical composition and another chemical composition. In alternate embodiments, the chlorinator/sanitizer is configured as a parallel treatment unit and as a series treatment unit. In the series treatment unit, the full flow through the chlorinator also flows through the sanitizing media in the santizing canister, so that all the water is both chlorinated and sanitized by the chlorinator/sanitizer. In the parallel treatment unit, the flow within the unit is divided into two streams which are separately treated. These two separately treated streams are then reunited and mixed together in the outlet chamber so that the water is both chlorinated and sanitized by the chlorinator/sanitizer.