To cite a publication with two authors in a presentation, should I write "X and Y" or "X et al."? I am preparing to give a presentation after few days. Some papers are authored by two people and some are by three. While referring their work during presentation what should I write for two authors. 

Suppose there are two authors only - X and Y. Shall I write X et al. proved that ...... or X and Y proved that....
This was addressed on English.SE, Is "et al." acceptable for citations with exactly two authors?

The answers there indicate that major citation styles (MLA, Harvard) do not use "et al." when there are only two authors.

APA style also does not use "et al." for only two authors.

I'm not going to exhaustively check every citation style, but I'm not personally aware of any where "et al." is acceptable when citing a publication with only two authors.

Also of interest to those who happen to like grammar: the English.SE answer also points out that using "et al." for two authors can be considered incorrect, regardless of style considerations (text in brackets is added by me):


  The Latinate abbreviation "et al." is short for "et alii," which means, "and others," and always refers to people, not objects. So if you had two authors, adding "et al." would indicate that there were [plural] other authors  - and since there are no[t multiple] other authors in this case, it is incorrect to use it.


While "et al." could also technically stand for the singular "et alia" which would be technically correct, that's certainly not a conventional abbreviation.