Can multiple helicopters be used to lift a heavy load?
This video on YouTube apparently shows that it has been done at least once. A rigid beam was used to counter the forces that would draw the two helicopters together.

Screen grab:


I also found this fascinating report (.PDF link) from Piasecki (now part of Boeing Defense, Space &amp; Security) that studied the feasibility of attaching two CH-53D helicopters to each other, including interconnected powerplants and flight controls!