These two verses are treated together because they form a chiasmus. In this case the elements form an A-B-C-B-A pattern in which the A and B elements are the words mouth … lips … lips … mouth …. The rhythmic structure of the two verses is shown most clearly in the poetic layout used in New Jewish Publication Society’s Tanakh (with the last line not shown):

Quoted with permission from Clark, David J. & Hatton, Howard A. A Handbook on Malachi. (UBS Helps for Translators). New York: UBS, 2002. For this and other handbooks for translators see here .