Interesting idea, but great to see that they made it work! I've seen the jet-assisted Waco at an airshow years ago, this one will surely be spectacular too once it's completely finished.
As a taildragger, would it be more or less prone to ground looping than a single one?
Difficult to say anything about that, depends a lot on the CG location, which will most likely be near the original location to keep things manageable. The only thing that comes to mind is that the additional width of the combined undercarriage might be somewhat beneficial.