If there are two authors, the Harvard style requires you to cite both authors in the text and include both their names in the reference list.
Your text will look like this:
Parslow and Jorm (2003) have found no evidence that pet ownership is associated with cardiovascular health benefits.
And your reference like this:
Parslow, R.A. and Jorm, A.F. (2003) Pet ownership and risk factors for cardiovascular disease: another look. Medical journal of Australia. 179 (9), 466-468.