That's because the WLL is just fine. It was stated that these things weighed 900 lbs apiece so 7200 lbs total. With 2 chains you are covering the WLL requirement by over 100%
We haul a lot of Genie scissors out of North Bend. My company is ok with using one chain for multiple units but insists that the chain goes to the deck between each lift. The problem I've had doing it this way is getting all the slack out of the middle of the chain. Your way at least took care of that problem.
Those do not weigh 900 each. The first one in the picture is a Snorkel Lift s3219e, you can see the model number. The gross weight of that model is 1564 KG= Just over 3,000 pounds.