Two publications by the same author

If using two publications from the same author published for the same year, the references list entry should be written using the same citation as for a single author along with a lower-case letter to indicate the separate publications. The letters are assigned based on the order in which the citations appear in the text. 

Your in-text citation may state:

The references list entries in your list show the entries in the order in which they appear in your text: