When you need multiple citations to appear at the same point in your text – for example, when you refer to several sources with one statement – you can present them in the same set of brackets, separated by semicolons. List them in order of publication date:
Direct Citation
Martin (2005) and Rothfuss (2011) both infer that...
Indirect Citation
Several in-depth studies have investigated this phenomenon during the last decade (Singh, 2011; Davidson, 2015; Harding, 2018).