The late Art Stump made a very small batch of Bugatti "replica" frames, with more modern geometry. He spent hundreds of hours on them each. A quad tubed main triangle and fork.

There have been a number of twin seat tube or downtube frames over time. the Rigi comes to mind and the Colnago Bi-turbo or something like that, a twin down tube ti frame.

I drew one up, problem is suitable tubing, fixturing.