Obselete Code Inherited (reverse engineering? having two dlls with the same name in the same project?)

we've inherited a c# application that has code behinds and dlls that are obselete.  The dlls however have the same versioning as the version without the code behinds, which is up to date.  What is the suggested work around?  I cannot even add the dll's to the project to give them a separate alias without an error!