I have a VivoFit3 that I use for step counting only (Because battery life + I wear it on my hip for more accurate step count).
I also has a VivoSmart3 that i use for 24/7 heart rate logging.
The VivoFit is set as "Preferred Activity Tracker" in Garmin Connect, however, the Vivosmart keeps overwriting the step counter in the app, and when I (manually) sync the Vivofit that value is then synced down and overwrites the internal step counter in the Vivofit.
I would expect the preferred device to override the other one, but maybe I'm missing something?