The levels have gotten a bit tricky now, so I'm sure you'll only need a hint instead of a solution here.
I'll assume you've already cleared one of the three gates on the right. As soon as you move that box to weigh down a button, there's no real way (nor any need) to move it again; so you must choose which makes most sense. Both character's arms are long enough to do what you need. At least that bottom character won't be needing to move that box.
The hard part is knowing that all three buttons need to be weighted down somehow. You've done the first and can work out the second, but how will you get the third?