You can turn on the Chart Toolbar, then select the chart element you want to format from the drop-down. That comes in handy when you can no longer see a chart element, because the existing formatting makes it invisible. Then click the Format icon or the Format menu - Selected <chart element> to open the format dialog for that chart element.

As long as you set the same parameters for the Y axis and the Secondary Y axis, you should be fine.

cheers