In the beginning, I connected my watch to MayMyRun and they will sync automatically after the workout. I connected my watch to Strava yesterday without disconnecting MapMyRun. I assume both of them will be updated after the workout. However, I can only see the workout on Strava but not on MapMyRun. How can I have the watch upload the workout to both websites automatically after the workout?
If you mean you are uploading from your watch to Garmin Connect, then from Garmin Connect to Strava and MapMyRun, there is another way of doing it if the automatic syncs don't work. You can use a site like https://tapiriik.com/ which will sync your run over many different sites, including Connect, Strava, and MapMyRun.