Rebuild the second solution in Debug mode on your own machine (so that file paths in that PDB are specific to your machine).
Copy both the .DLL and .PDB files to your references folder. Visual Studio will pick up the .PDB file automatically and use the file paths to show source.
You can also use Symbol Server and Source Server to achieve this when the referenced assembly is built elsewhere: http://msdn.microsoft.com/en-us/library/vstudio/ms241613.aspx