Deni,
There is a convoluted history to it, but essentially, no. The two sites are NOT mirrors. DCM was founded by former members from GAC, and the two sites membership rosters would probably have 80% matches.
How it got this way is not as important as the fact that both exist and benefit each other by virtue of sharing the task of distributing this stuff to the public. Both are non-profit sites, and each has had issues in the past when, periodically, the sheer volume of stuff flying out the doors will bring the site to its knees (this is why we INSIST on no simultaneous downloading, and ask that you only download in one session what you intend to read for the day). In these instances, the redundancy of the sites is a tremendous comfort.
You will see variations in what each site carries, based on management styles. We both try to make sure that everything we offer is legitimately Public Domain and legal, but that is tricky business and subject to interpretation.
So, if you post (thank you and God bless you), do post to both sites.