The key thing that will enable or prevent knowledge sharing is compatible content licensing. E.g. if you and another project are both on a CC-BY-SA[1] license (the license used by Appropedia), you can share content in both directions, which is perfect. This license allows for commercial use, avoiding any uncertainty over use with advertisements, and is compatible with various other sites & /projects as well. Someone could adapt an Appropedia page, adding their own ideas, and then those additions could be used on Appropedia or elsewhere - with attribution back to Creative Citizen.
For complete beginners
For someone completely new to Creative Commons, a great place to start is the Get Creative video. There are more videos here.
Then you can Meet the Licenses, or read An Introduction To Creative Commons.
Those links don't try tell help you decide which content license is right for you. Part of the reason we use CC-BY-SA rather than using the NC (NonCommercial) clause is found at Non-commercial licenses vs open licenses. A much more extended argument is found at The Case for Free Use: Reasons Not to Use a Creative Commons -NC License.
If you're happy with the same license, then you can talk with your potential partners about details, and technical issues, such as how to ease the process of sharing between the sites.
Public domain vs Creative Commons
The most extreme form of "open license" is to use no license at all - i.e. public domain. See the Public domain page for arguments against Creative Commons from a pro public domain perspective.
If you do choose public domain, you can use a "public domain mark" from Creative Commons (or elsewhere?[verification needed]) to let search engines find and index it.
