Exciting video!
The Mavic can carry, for a short time (
battery discharge over-current), even 900g but the weight must be centered while in the video are very loaded the rear propellers, IMHO, this the reason of the loss of altitude.
The maximum payload that can carry without "
battery discharge over-current" error seems to be about 750g ... if the payload is centered (obviously, no wind), a Mavic can carry another Mavic