I'm not an expert on databases, but from what I remember, having multiple people edit the same entry is very tricky.
Also backing up and restoring a database, although possible, may not be practical. What if the initial creator creates a backup and passes it to the second GM, but the second GM is slow in making his additions or changes and getting the file back to the original GM? Then, what if the original GM gets frustrated and starts making additions, corrections, and other changes to the version of the database he has? It could quickly lead to problems.
The only practical way, in my opinion to have two GM's working a single campaign would be to have one do all the data entry while the other brings over notes or sits down next to the guy that has the master copy of the database.
Now, somewhere down the road once we are able to push our work up to the repository and make it available for others to look at, we can hope for a feedback and suggestion capability. This function should allow people to contribute notes and other feedback on parts or the whole of a project. The original creator of the content could then look at the proposed feedback, notes and suggestions and decide if they are worth adding to his or her work.