Found the solution. For reference to those who have the same question :
add an e-chart-axes tag
for each y-axis you want add an e-chart-axis tag, set the name attribute
use that name attribute as the value for the yAxisName attribute of your data-serie
<ejs-chart id="byagechart" width="100%" palettes=@palettes>
<e-chart-primaryxaxis valueType="Double" labelFormat="y" edgeLabelPlacement="Shift" interval="5" minimum="0" maximum="75"></e-chart-primaryxaxis>
<e-chart-axes>
<e-chart-axis name="abs" desiredIntervals="10" minimum="0" maximum="@( yMaxByAges )"></e-chart-axis>
<e-chart-axis name="pcnt" desiredIntervals="10" minimum="0" maximum="@maxPcnt" opposedPosition="true" labelFormat="P"></e-chart-axis>
</e-chart-axes>
<e-series-collection>
<e-series dataSource="ViewBag.ByAges" name="No-Show" xName="PatientAge" width="2" opacity="0.5" yName="NoShowAmount" yAxisName="abs" type="@Syncfusion.EJ2.Charts.ChartSeriesType.Column"></e-series>
<e-series dataSource="ViewBag.ByAges" name="No-Show [%]" xName="PatientAge" width="2" opacity="1" yName="NoShowPcnt" yAxisName="pcnt" type="@Syncfusion.EJ2.Charts.ChartSeriesType.Line"></e-series>
</e-series-collection>
</ejs-chart>