Visual Studio Tools for Office
By design, Office applications are designed to open multiple files in the same instance. You could try doing what you describe by using code (VBA would do it) that starts a new instance and loads the document. Example:
Dim xlApp as Excel.Application
Set xlApp = CreateObject("Excel.Application")
xlApp.UserControl = true
xlApp.Visible = true
xlApp.Workbooks.Open "Drive:\path\workbook.xls"