The same way as you do the regular Draumr, the only difference is that the height of the straps, on the tree, have to be adjusted according to the bodyweight of the people using it. The strap has to be mounted higher up on the tree on the side where the heavier person is going to be sleeping.
You might have to account for some testing and trial and error.
Here is a video showing how to set up the Draumr Double. If you prefer written instructions, here is a link to our user manual.