I don't think that there is a way to specify more than one CSS class on a single HTML element in the rich text.
Given that these come from FormatAreaStyles.css, you could, for example add an option like
class1---class2 (giving you HTML something like
<p class="class1---class2">Content here</p> in your rich text source) and then transform this in your templating code (to
<p class="class1 class2">Content here</p>). This could be done using 'Find and replace' or a regex.
Obviously this doesn't scale well if you're looking at adding (combinations of) lots of classes, but would work for two or three classes on a limited number of elements.
Alternatively, you could look at using something like a Custom URL (on the field name in the Component), but this could become very fiddly.