Wasn't sure whether or not to post it as it's not a coaster as such, just a new take on an element. I've no idea if it exists either, so forgive me if it does.
For now, I've called it the 'Interlocking Duelling Roll
' as it's an ordinary barrel roll but is designed for duelling coasters. Basically, the two tracks/sides line up (the trains running along them side-by-side) and the two perform a barrel roll around each other. The two trains effectively roll over and under one another and create a unique near-miss effect. The element could be performed once, twice or three times to prolong the effect.
I'm sorry that the concept I've created to illustrate the element is rough (i.e. has no supports, isn't very smooth or heart-lined and the cars don't properly line up with one another), I just wanted to make a mock-up of the idea I had floating around in my head.
Let me know if anything like this exists or if you'd like to incorporate such an element into one of your own coasters as it'd be cool to see a more refined version in action.