The easiest way to do this is to first change your taskbar settings so your Excel files will show as separate entries on the taskbar. To do this right click on a blank spot of your taskbar and choose Properties. For Windows Vista uncheck the box that says Group similar taskbar buttons. For Windows 7 and up where it says Taskbar buttons, make it say Never combine from the dropdown list.
Now open Excel by itself from the Excel shortcut rather than double clicking an Excel file. Drag the empty Excel window to your secondary monitor and then open Excel again and keep in on your primary monitor. Now open your files from the Open option within Excel.