Caroma makes a dual flush one-piece regular bowl toilet.
Toto makes the Aquia two-piece in elongated, that looks very much like a one-piece.
Both of these have fairly high tanks.
It's going to be hard to find a low tank dual flush. The lower the tank, the worse it flushes.
You can't get good "scores" with a low tank, unless you go with pressure assist tanks.
And at this point, that moves you back to a tank and bowl, bolted together.
That's good. Most product out there is fairly priced.