A Dueling Flying Dutchman Coaster? That would be awesome! Imagine the two trains enter interlocking corkscrews like that on B&Ms with the riders on their backs as with Stealth. When riding one side you'd see the other side, only they're at a right angle to you! Like being in a Rotor or Centrifuge.
How about a Floorless vs. Inverted Dueling Coaster? A section would be the Inverted zooming over the Floorless really close & the riders on the Floorless think they're gonna get clobbered in the head!
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Home Park - Wonderland Sydney, Australia.