I do not follow. I have two excel documents open. Each is presented to the user like an independent excel app. So in my windows taskbar there will be two excel icons stacked. If I hover I will get a quick preview of all of the excel instances that are open.
If I close one document the excel instance that is running that window will close too. The other instance of excel with its document will remain unaffected.
Have you not noticed all the comments complaining that Macs see closing a document and closing a program as two separate things, while Windows still does not?
Edit: It wasn't until Office 2007 that some Microsoft apps (but not others) started breaking the Win32 MDI UI.
If I close one document the excel instance that is running that window will close too. The other instance of excel with its document will remain unaffected.