I am trying to do something which seems simple but cannot for the life of me find a website/software to do it.

Many are familiar with the Strava Compare feature or Strava Flyby.

I want to do the same thing but with any two activities. Let's say they were done on different days and on slightly different routes. This hypothetical software would show both GPS tracks on a map, and there would be a scrollbar or something for elapsed time. It would assume both activities theoretically started at the same time, and show you where both people are as time progresses. Like Flyby, but for activities at different times. Does this exist?

Or, is there a simple tool to move the times of activities to force a solution using Strava Flyby?

Thanks!