Try turning on Preferences / Plugins / ActiveX Document: Create temporary copies of Office files.
That may allow you to view a file in the viewer while also loading it into a separate Excel window, since it copies the files to your temp directory with a new name before opening them. Of course, it will break macros etc. which depend on file names or other files in the same directory, but those are uncommon.
The problem you're seeing isn't specific to Opus. All Opus really does to view an Excel file (via the ActiveX route -- the MultiView plugin is different) is launch Excel itself within the viewer pane.
You'll see the same thing happen if you try to open two instances of Excel and load the same file into both. It's all part of the wonderful design of Microsoft Office.
Other ways around the problem:
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Close the viewer before opening the file. (Of course.)
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Configure the ActiveX plugin so that it doesn't handle XLS, DOC, etc. files and instead use the MultiView plugin. This requires that you install some DLLs which you can get for free with a number of other products, but once installed the Opus viewer can display Office documents without using Office itself which means the real Office applications don't trip over themselves.