So, you would like the citations to appear like “Smith (2009, 2012” within the text? If so, you will have to configure both citations into the group to “Display as Author (Year).” Click on the group of citations and then click Edit & Manage Citations. For each of the citations in the group, choose the option “Display as Author (Year).”
My apologies if my screen capture was not more specific. If you click on the Citation and then go to Edit & Manage Citations, that will take you to the same dialog as right-clicking or control-clicking (for Mac) on the citation and selecting Edit Citation(s) > More.