Quoting from 2-author work; Subject-verb

[Manitshah]

A book has been written by two authors, Itule and Anderson. It was published in the year 2000. I want to use an idea I got from their book.

Here's the problem sentence:

"Itule and Anderson (2000), advocate that, in a democratic community, news functions........"

Should I write:
Itule and Anderson (2000) advocate

or

Itule and Anderson (2000) advocates

??

I'm not sure whether I should refer to the TWO AUTHORS (in which case I would use advocate) or to the BOOK (in which case I would use advocates). Note that I also have to mention the year of publication.

[Mister Micawber]

.
I have seen both methods in reputable books and journals. I think that it is simply a matter of how the writer is viewing the reference-- as multiple authors or as one book. As you use the verb 'advocate', a quite human activity, I thought first of two people and expected the plural verb.
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Native English teacher at Mister Micawber's