The two different series are two different visual elements. When placed together in a chart it is expected for the second series to cover the first one. The same thing happens if you place two Buttons on top of each other - if the first button is smaller, you would not be able to see it.
One option you have is to determine which area series is "smaller" and set its ZIndex to some number (say 10). This will result in the "smaller" series drawn on top of the other series.
Another option is to use a different type of display - stacked area series or simply line series. Or perhaps you can set a transparent fill to both series, so that they will surely be visible.
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