First, install gpsbabel and (optionally) gpxviewer.
sudo apt install gpsbabel gpxviewer
Now, use gpsbabel to combine the two .fit files into a single .gpx file.
gpsbabel -i garmin_fit -f B3781326.FIT -f B3792523.FIT -o gpx -F merged.gpx
gpxviewer can be used to check the merged activity on a map.
The merged activity file can now be uploaded to Strava using the “Upload activity” menu option.
The data in the merged file will be as if the watch was paused at the end of the first recording and restarted at the start of the second activity. Hence, the intervening time will be included in Strava’s elapsed time for the activity, but will be excluded from the moving time. To fully merge the two activities so that there is no discontinuity, the .gpx file can be edited in a text editor. Simply remove the closing </trk> tag of the first activity and the opening <trk> tag of the second activity, which will appear on successive lines, then save and upload to Strava.