You won't be able to bind the code (solution and projects) to 2 different TFS servers. If you're using the online version of TFS what reason would you have of also trying to store the code in a local repository?
You definitely won't be able to merge the code between the different servers. TFS is a centralized version control system and merge operations need to be handled and tracked by the server.
You could theoretically Manage the code in one of the TFS servers (i.e. have the solution bound to TFS online) and manually manage the code in the other server. However TFS 2008 only supports local workspaces which means that it will make files read only if they are not explicitly checked out. This will cause you no end of pain.
Even if you did it the other way round (made the local TFS server the "master") you will almost certainly create yourself a load of problems as TFS simply isn't designed to be used this way
Also TFS 2008 is pretty ancient and unsupported.
EDIT:
The typical TFS workflow is.
- Create a development branch on the server.
- "Get" the code to a local folder on your dev machine (called a workspace).
- "Check out" the files you need to modify from your branch or "Add" new files as needed.
You should regularly "check in" files to your branch to make sure your changes are saved on the server.
When your changes are complete then "merge" the code from the development branch back to the Main branch.
The code is stored and versioned on the server, but your changes are made on your local machine. To interact with the server you use the Team Explorer plugin in Visual Studio. Modern version of Visual Studio have Team Explorer built in.
If you're using the online version of TFS then you'll want to use Visual Studio 2012 or above.
If you're planning on Forking the code and do not plan on committing your changes back to the server, then you're probably better off using something like Git. You could use Git-tf or Git-TFS to pull changes from TFS and then that would make merging a lot easier and you could use your local Git Repo to manage your own changes