I second this... the easiest way is to combine the values using a processing rule....
However, IF you have the possibility that source or medium may not be provided along with the other one, you could also create more complex rules in Adobe Launch to compensate for that and fill with "none" if you so desire....
So:
If neither provided, leave empty
If Source AND Medium is provided = "source value / medium value"
If ONLY Source is provided = "source value / none"
If ONLY Medium is provided = "none / medium value"
Technically you can do this with processing rules, but it's a lot easier with Javascript or a concatenation extension.