Thanks for asking about this. I was able to create a binary system between Earth and the Moon by doing the following:
1. Adding the Moon orbiting around Earth
2. Pausing the simulation
3. Increasing the Moons mass to be equal to Earth's
4. The Moon is still listed as orbiting Earth, so I went to the Moon's Motion tab, set the Eccentricity to 0, and then locked the Eccentricity
5. Adjusted the Semi-major axis to the desired value
6. Selected both the Earth and Moon and created a Barycenter, then focused the camera on that Barycenter

Played the simulation at a couple hours/second. This system was stable up to a few months/second.

However, since both the Moon and Earth are orbiting the Sun and the Sun has the greatest gravitational attraction to both the Earth and Moon, they will both be listed as planets orbiting the Sun.

We're working on some improvements to our orbit system and the way you can adjust orbital parameters that should make it easier to create binary systems like this in the next update.