Select multiple Refiners in the same category (part 1)

The selection of multiple Refiners in the same category is possible out of the box.

For instance if we select the Word refiner in the Result Type category it will show us the following url:

According to the Keyword Query Syntax Reference Multiple Property Restrictions work like this: When you use multiple instances of the same property restriction, matches are based on the union of the property restrictions in the keyword query. This is functionally the same as using the OR Boolean operator.
So if we just add "%20fileextension%3D%22pdf%22" to the URL it will get us Word OR Pdf documents as a result.

The nice thing is that the XSL recognizes both refiners as selected.

Even the deselection of a selected refiner is working.
In this case, if we would deselect the Word refiner, the url is correct:
http://sp2010/SearchCenter/Pages/results.aspx?k=sharepoint&r=fileextension%3D%22pdf%22
But on the other hand if we would deselect the Pdf refiner the logic is broken. When multiple instances of the same property are used -like Word- the url is not correctly processed: the spaces (OR) are replaced by AND. This will show us no results of course.

Another example of selection of multiple refiners is with the Author category. The author can/will show us 2 names: the original author from the document property, and the one that created the document (by upload).

First we select the original author of the document.

Then we select the creator of the document.

The deselection of a selected author is working fine.

Conclusion:
So the selection of multiple refiners in the same category is possible but the UI doesn't provide the full and correct navigation for it.

In part 2 I will try to overcome this.