They may not appear if there's an X axis mismatch. If the one series uses
dates and the other uses categories or another sequence of numbers, the
second series may be out of scale from the first. Another reason the second
series may not be visible is if the Y axis is fixed based on the first
series and the second series' Y values are within the range of the axis.
Paste a sample of the data into a response so we can advise.
- Jon