Calorie burn from fitbit vs garmin

srredd1

I have been using my garmin forerunner for excercise but recently purchased a fitbit HR. I have noticed a huge calorie difference between the two devices- mainly because the workouts I do tend to be HIIT and weights. Can I sync both devices to my MFP account, or will it double the calories burned? Or is it better to manually add the difference? Thanks!