Get bart past the 'stamps' and have him grapple onto the first stand.
Using the Hand of Buddha (up the really tall ladders opposite), move the square 'target' platform from the floor to the stand closest to Bart. Once it's in place and the platform has moved, switch back to Bart and have him grapple over to it.
Switch back to Lisa and have her move the platform Bart has just left onto the next stand, and then have Bart grapple over to it. Repeat this process until Bart's within reach of the lever, and have him glide over to it and pull it.
This will uncover the two red buttons in the centre of the floor. Stand on them with Bart and Lisa and the next pipe will open
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Let me know if this helps
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Phenom