I have done this before and we had two "seesaws" built that we painted black to match the back black curtain. We had to move them a bit downstage to make this work, but it was effective. The two actors sat on the end in makeshift seats with "handles" on the side, mostly for them to feel as if they could grab onto something, and even had black belts around their waists to hold them in place. The techies, in black, moved them up and down during the scene. The fulcrum is key here. The closer to the actor, the less you had to push down in the back, but then they don't go very high. The more height is achieved by moving the fulcrum to the back, but then you have to have more techies to push down. They didn't go very high, but we had them on small platforms as well so we could move them back and forth in a straight line and the whole effect worked well. We also used follow spots to go up and down with them and bubbles from bubble machines all around. Hope this helps.
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