A mixte is a mixte regardless if there are two tubes running between the head tube and seat tube or one -or 3. It doesn't matter.

As far as the engineering/design is concerned it is one solid support brace running from the headtube past/through the steat-tube (via a lug such as this) and on to the rear dropouts, bisecting the rear triangle.

How it gets there doesn't matter. That's just cosmetics and an exercise in choosing different tubing shapes, sizes and lugs by the builder -not something considered in the definition of a Mixte.