I am planning a trip between two cities (City A and City B) and want to introduce a stopover of a few days in a third city (City X). I don't know which third city to go to, I want the search engine to give me all possible options where city X could be (eg, San Jose, San Francisco, etc.). To minimize travel friction, I want both legs of the journey to be strictly non-stop flights (i.e., non-stop from A to X, and non-stop from X to B).
Most flight search engines I looked at such as Google Flights and Skyscanner are optimized for single-ticket layovers or multi-city routes based on airline alliances. They don't easily allow you to reverse-engineer the intermediate destination by searching for the "intersection" of non-stop routes. E.g. , if using the multicity option on Google flights, one must specify at which airport the stopover occurs: instead I want the flight search engine to list all possible airports for my stopover.
How can I easily find an intermediate city for a stopover where both legs are strictly non-stop flights?