Two (or more) time series can be combined using the Operations=>Combine Series item. Selecting this item creates a Combine Time Series popup window. Simply select the two series you wish to combine in the popup window and hit the Ok button.
Here (above) we see two series which have combined into a third series, this new series is independant of the originiating series and can be manipulated like any other series.
FROG - A Time Series Analysis Package