When a publication has one or two authors, you should cite:

All of the authors' last names and the date of publication every time you refer to this item in your text

All of the authors' last names and the date of publication the first time you refer to this item in your text and only the first author's last name and the date for subsequent citations

The first author's last name followed by $"$et al$."$ and the date of publication for the first and subsequent citations

All of the authors' last names and the date of publication the first time you refer to this item in your text and only the first author's last name followed by $"$et al$."$ and the date hy subsequent citations