Citation: refer to author or paper? What is the British English way of referring to a source in-text.

Is it plural because there are two authors, like this:    


  X and Y (2011) describe ...


Or is it singular because you are referring to a singular source, like this:


  X and Y (2011) describes...


So what I am asking is whether you refer to the source or to the authors?
Supporting @user153812, I'd write


  Marx and Engels (1888) propose sharing property ...


but


  The paper (Marx and Engels, 1888) provides the first definition of communism ...


The rule of the thumb is that a citation in parenthesis should not disturb the flow of the sentence. Every time this not happens, I find myself referring to the authors, hence the plural. The only exception is when there is a sole author.


  Lenin (1907) provides a crucial extension of the initial suggestion of Marx ...


I had to correct myself, having written it initially in past tense, which sits better with my belly felling, but does not provide a good reference to the genus.

(Note that I did not proof-check the dates, it's just the May spirit.)