I believe these two are closely related, and we should discuss where to draw the line between the two, should both sites be launched around the same time. I'm going to include my proposal as an answer.
I would propose this.
- Anything that is pure science, especially on a large scale (IE, observational, beyond the solar system, etc), belongs to Astronomy.
- Anything that is science on a small scale (Probes, observers, etc) could belong to either site. IE, understanding what is happening at the one specific point that the voyager probes are crossing the heliosphere.
- Anything related to the design, flight, and other technical details of spacecraft belongs to the Space Exploration and Technology.
- Questions relating to the optics and optical chains of space telescopes could go either way. Same goes for non-optical observational instruments in space.
As @lyncas has commented in response to the question, there is a lot of merit to merging the two, rather than try and differentiate between them.
Together they offer more critical mass, which may make the difference between either (or both) not surviving as a standalone site.
There is likely to be enough overlap to keep most followers happy.
I suggest there is an important boundary between Astronomy and Space Exploration, and the two should be separate if at all possible.
The important distinction is what we can know (Astronomy) versus where we can go (Exploration). I believe every question will clearly belong in one or the other, with little synergy from being together.
Astronomy reaches to the limits of the light-cone, black holes, supernovae, galaxies, galactic filaments. Or to the big bang, or to a future of accelerated expansion resulting in a one-galaxy sky. Places we're hardly dreaming of going. It's also concerned with the instruments of astronomy, land-based or orbiting telescopes, x-ray or radio astronomy, but the point is they are reaching further than we have any practical expectation of ever going.
Exploration includes rich social and biological considerations, multi-generational spaceflight societies, terra-forming, the physiological effects of weightlessness, and intractable health threat of cosmic rays, the orange palms of Biosphere 2, etc. It also includes robotic exploration, a physically wider issue but only slightly. As interesting as this is, it is comparatively, extremely short range.
Finally, even if there is insufficient interest today to combine them, the safety-in-numbers falls apart: There will be enough interest in each individual topic eventually and at that time separating the two will be tedious. Better start now, or as soon as enough gather around.
Finally, a word of caution against too much merging of what sounds similar, or else we'll all end up in mathematics.