Final answer:
For two authors, list their last names with an ampersand as 'Simpson & Dumas' for citations and references, but use 'and' when writing the authors' names in a sentence.
Step-by-step explanation:
When citing a work with two authors, their last names should be listed in the following format: Simpson & Dumas. It's important to use an ampersand (&) rather than the word 'and'. This format applies for in-text citations and references in various citation styles, including APA and MLA. For example, in a reference list, you might write Kristoff, N. D., & WuDunn, S. (2009). If you are including the authors' names in a signal phrase in your writing, you would use the word 'and', as in 'Kristof and WuDunn explain in their preface...'