Hello JK
I am not surprised that you are having some difficulty with a twin axel. The problem is compounded by the interactions of the suspensions of both axles, which depending on the angle of the caravan can add or detract from the nose weight figure
Despite this the same basic approach is required for both single and twin axle units:
To get an accurate reading of the nose weight, the caravan must be on level ground, the wheels chocked (so it doesn't roll away) and the hand brake off.
The tow hitch must be set to the same height as when it is attached to the car, (so the caravan its self may not be level) as changing the height actually does change the nose load.
The jockey wheel must not be in contact with any surface as this reduce the nose load at the hitch.
A stick (cut to length) and bathroom scales is a reasonable solution. The stick must be vertical and under the hitch when making the measurement.