Connecting the two in a zip-line fashion? I don't know enough about materials science, but a 1/4" welded steel chain has a working load of 3150 lbs and weighs

Connecting the two in a zip-line fashion? I don't know enough about materials science, but a 1/4" welded steel chain has a working load of 3150 lbs and weighs 0.63 lbs per foot. 1.5km is roughly 4900 feet, a chain that long would weigh 3100 lbs. So it would hold until the first wind or rain -- keeping in mind that these specs are for a modern forge-produced chain.