Person a starts in A and is going to meet person b in C. Person b starts in B, meeting up with person a in C.
Is there a service that can find such two flights without too wait time in C?
Google Flights allow one to specify several departure and/or arrival airports, separated by commas. E.g., if I enter "CDG, JFK" in the departure field and "NRT" in the destination field, the results will show both CDG-NRT and JFK-NRT itineraries. From there it should not be too difficult to find two trips whose arrival times are close.
I don't believe that there is such a service, but there is an indirect way of accomplishing the same thing.
You can search for flights on various websites (Google, Expedia, etc.) and then search by arrival time. Run the two searches in separate browser windows or tabs, so that you can display them simultaneously or easily switch between them. Simply search through the arrival times by starting with a convenient time of day and then seeing if there are flights in both windows with similar arrival times.