100C is overkill. A Mavic drone does not pull more than 25A even in the most difficult moments. A 45C battery is more than sufficient. The important is to try to match mah from the main battery.I have done this....you will need a battery with more like a 100c rating or the Main battery will take the brunt of the load and you won't get the extended results you want.
I ultimately decided the battery performance was too erratic and stopped using it for fear of losing my drone.
The more weight you add, the more difficult to control can be the drone. Make sure to locate correctly the CG of the drone. Also the more weight the more effort the motors will push and internal electronics can be overheated.
I find such a youtube video:Don't!
This is a LiPo battery, the MA2 has LiHV batteries.
Buy a LiHV battery (it will be rated 11.4V instead of 11.1V)
Otherwise when both batteries are fully charged and you parallel them, the MA2 battery will be at 13V and your external battery at 12.6V. If you immediately start flying, then probably you won't overcharge the external LiPo, but I wouldn't do this routinely.