I know of a 6ft wide x 4ft thick branch-bridge about 75ft up between two redwood trunks (probably reiterations of the same tree genetically). Coast Redwoods do this bridging more than any other NW conifer I know of, but I have seen maples fused to doug firs 8 ft up, and doug firs to sika spruces 40ft up.
The link is to a company (no affiliation) that does it, but the pictures are interesting to see. Since white willow will fuse just like cedar I wonder if you could make it over a wider distance by planting support beams/trees.
She changes everything She touches, and everything She touches changes.