Garmin Basecamp usually works. I often use it for splitting or cropping FIT files from my Forerunner. Though it may lose some of the fitness data, eg laps.
Or you could upload it to Strava, then crop the activity there. Then upload it again, then crop the other part. Note Strava won't let you upload duplicate activities, ie if they have the same start time. So for your first upload, you would need to crop off the first part of the activity.