Right click on your chart and select Chart Type > 2-Axis
Or initially before creating a chart, select the chart type at the bottom that looks like a bar and line chart together with ticks on both the left and right.
This creates a dual Y-axis chart and now the Series and Axis details in the chart Inspector show more details. By default the chart will have 1 line and multiple bars, and you can select a series and in the inspector and change the bars to lines by clicking on Series Type > Line. You also have options in the inspector for which axis to plot on.
In the past, I have used Excel to create a chart with two y-axis, then copy and paste it in to Keynote. Not as beautiful and simple, but does the job. To do this in in Excel Mac 2011, after you have plotted all your data series in a chart, right click on a single series, select Format Data Series > Axis > Plot Series on Secondary Axis.