How to add a citation to existing string of citations

Hello, Using 'Cite while you write' how can I add a new citations to an existing string of citations so that it automatically blends in? For example if I want to add a new citation to "(Lu et al. 2010; Zuo et al. 2011) " I can not find a way to do that apart from adding it at the end which looks like this "(Lu et al. 2010; Zuo et al. 2011) (Newmaster et al. 2013)" instead of "(Newmaster et al. 2013; Lu et al. 2010; Zuo et al. 2011)"? If I want it the latter way then I would have to delete the existing string, highlight all the citations I want and insert them anew. Surely there must be an easy way to do that .... This question has been asked before (http://support.mendeley.com/customer/portal/questions/842279-add-citations-to-existing-citations), however, wasn't answered satisfactorily. Thank you Markus