That one almost drove me mad. It's going to be hard to describe but the way that worked best for me was to put both boxes together over the sliding block. I then dropped one down just out of it's range and the other as much on the tip as I could. Then I let it draw back for just a second before I hit the gravity to make them go back up. I then timed the sliding block so that I landed the right most box on top of it on the edge and as it came back I hit gravity again.
I have had more success though with this method. Get the boxes side by side on the ceiling and put a small gap between them. Drop them with the right most box hitting the edge of the sliding block so that it tips over on top of the other box. Then you can use the sliding block to keep them on top of one another while you climb over them and then push them back to the gravity trigger.