Note: this particular test site is meant more for folks to do quick test drives of the Known platform rather than serving as a platform in the way you’re describing. As an example of what you may be looking for though, here’s an original post on my own website (note the “also on” link at the bottom) and here’s the copy that was syndicated into the separate “community service” on an entirely different domain.
I suspect you could use other sites/services like WordPress to do something like this as well.
Alternately, you could have folks post on their own site and aggregate things in a “planet-like” fashion via RSS (by keyword perhaps) or other means on a central hub as suggested by Aaron Parecki.