Currently, there is no way to disconnect a series from a particular measure, especially as it pertains to combination chart. It is simple enough to create a combo chart with a series and several measures and allow that series to apply to both measures. In the case of a line/stacked column combo chart, you will end up with a stacked column as well as several lines for each class in the series.
However, there is no way currently to disconnect one of the measures from the series in order to create a single line rather than multiple lines per series' class. The only way currently to create an individual line is to create multiple fixed measures per individual class (to create a stacked column) and then a separate measure for your line. This does not provide for dynamic growth over time however, if our company were to add a new class (i.e. another stack in the column). Such a case would require me to manually return to the project and add that class as a new measure. Scalability is hindered in this instance.