It wouldn't be 100% wrong, but if I've understood you correctly, it would be better to apply "if" (or "when") to both subordinate clauses: "We use this phrase if / when we don't know someone and we haven't met them before." The structure is known as the "zero conditional", if you'd like to google it.

Hello! Even though both of “where” and “when” are questions .. you can use when for the sentence you provided above: “when we haven’t met them before” which will mean something like “even though we haven’t met them before” Hope that helped !

We use this phrase when we haven't met someone before. We use this phrase with someone we haven't met before.

Where is a reference for question pertaining to a place. Eg Where are you from? Where did you go after work Where were you when dad arrived? When refers to a time, day or year When did you arrive at school? When did they leave? "they left on Monday". When did you retire from teaching? "I retired last year."

When as a question word We can use when to ask for information about what time something happens: When did you leave? When are you going on holiday? When will you know the result of the exam? We can use when in indirect questions: She asked me when I would be ready to start the job. I wonder when the new computers will arrive.