I have two Visual Studio 2005 solutions, one of which builds a binary and all of its dependencies, and one of which builds a web app and some utilities and an installer for
Try selecting all of the project nodes in the tree then drag & drop them into the other solution.
If you want to automate the process you can try playing around with this tool
http://code.google.com/p/merge-solutions/
There's a way to do this built in to Visual Studio. Right click the solution, and select Add - Existing Project. Select the other solution from there.
You might want to consider using references instead of the dependencies. Those are stored in project's file and consequently are preserved across solutions. There might be something which can be done with Dependencies and not with References, however I am not aware of any such situation and for simple uses references are more than enough.