Sentence:  Where we once sent love letters in a sealed envelope, or stuck photographs of our children in a family album, now such private material is despatched to servers and clouds operated by people we don't know and will never meet. Perhaps we assume that our name, address and search preferences will be viewed by some unseen <WSD>pair</WSD> of corporate eyes, probably not human, and don't mind that much. 
Meaning of the word marked with <WSD> tags:  Two similar or identical things taken together; often followed by of.