Spruce tree grafting
In your opinion, is it possible to graft two mature spruce trees with a horizontal spruce pole or poles around 30 feet in length to act as a support for a tree house, while keeping alive all of the trees including the horizontal poles with limbs still on the bottom side? If so, what would be the ideal way to connect them?
From your description I would say this is not a viable plan.
Best of luck.