I assume you want to say that the download has finished, but the installation will not start/complete. You cannot install Project x64 alongside office x86, you'll need to either use Project x86, or switch to Office x64.
Side-by-side installations No support is provided for side-by-side installations of 64-bit and 32-bit editions of Office, including across applications. For example, there is no support for side-by-side installations of the 2007
Microsoft Office system 32-bit with Office 2010 64-bit, or for Microsoft SharePoint Workspace 2010 64-bit and Microsoft Excel 2010 32-bit.