If you are referring to two sources by the same author, produced in the same year, you can distinguish between them by adding letters to the end of the year for both your citation and reference.
Example: Research into the importance of chocolate on individuals’ moods has highlighted a difference between the cocoa levels of chocolate (Hoskin, 2011a, p.41). This has indicated that the higher the cocoa levels, the greater impact the chocolate has on mood (Hoskin, 2011b, p.12). However, further research is investigating how much of this is related to the sugars within the chocolate (Hoskin and Siddall, 2012, p.21).