I assume you are talking about an Excel Spreadsheet graph?

If so, what do you mean by a second Y axis label? Why do you want a second label on the Y axis?

As far as I know, there is no way in Excel to do this, but there are tricks you can use.

For instance, use the X axis label instead, they can be moved about, and the contents alignment changed from vertical to horizontal, or vice versa.

Use the main title in some way as a second Y axis label.

But other than that, I don't know any other ways.