Displaying two different types of data on the same chart make for easy comparison and can save space. However, while it may be tempting to compare two different trends on the same chart, using two different scales on the y-axis can be misleading, particularly if the scales are dramatically different.
Attention is also drawn to the intersection of lines, which may be arbitrary depending on the choice of scales.
If you are convinced that a double y-axis is the best way to display the data, use very different colors to illustrate the two different data sets. Color code your axes accordingly, to reinforce the data association. If your audience reads from right to left, put a label on your primary dataset on the left y-axis, and your secondary dataset on the right. Avoiding the same chart type for the two data sets can also help clarify the difference. For instance, instead of using two lines, perhaps combine a line for one dataset and a bar chart for the other.