Sentence:  Polyphosphates gave renewed life to soap products at a time when surfactants were a threat though expensive , and these same polyphosphates spelled the decline of soap usage when the <WSD>synergism</WSD> between polyphosphates and synthetic detergent actives was recognized and exploited . 
Meaning of the word marked with <WSD> tags:  the working together of two things (muscles or drugs for example) to produce an effect greater than the sum of their individual effects