According to this page on blooberry.com, you can specify multiple classes within a single class statement.

An example where this came up is at work; I needed to have a common element use two separate classes, "selected" and "navoption". I wasn't looking forward to creating a special class for every single element I wanted to use the effect on...

It's basically a space-delimited list. The format is as follows:

<span class="class1 class2 class3 etc">