URL search parameters…with ‘,’ and ‘ ‘?

I have been building my own custom ‘Advanced Search’ page and I have had some success in getting it to work. Basically I understand that when searching on tags and such that I can do an “AND” search by doing something like:

http://myblog.com/?tag=apple+orange

This will return all posts that are tagged with apple AND with orange…but leave out those tagged with one or the other only…or not at all.

I was reading a bit through Google searches on this subject and I came across this website: http://wpguy.com/articles/multiple-tag-category-queries/
which claims that one can do an OR search by separating the tags by commas, e.g.:

http://myblog.com/?tag=apple,orange

This should return results that are tagged with apple, oranage or both. This I cannot get to work though. Is there a way to do this through URL arguments or not?

Thanks!