This is a big nuisance with Microsoft Excel desktop version. I use Windows 7 and need to work with 3 separate Excel files simultaneously so I have them all open on my 2 screens. Excel by default opens them up ALL IN THE SAME WINDOW (wanna swear now, but keep reading).
To open Excel files in separate windows here’s what I do.
1. Open the first Excel document
2. Click on the Excel icon on the taskbar with mouse wheel, a new Excel window opens
3. Click and drag my second file into that window – Voila!
4. Do the same with the rest of the documents and I’m saved