The reason behind the reported issue is the fact that your business PanOffset and Zoom properties are defined in the context of the GraphControl. This means that when you create two controls, you will have different values of the corresponding properties that will not be synchronized out of the box. This is why you need to select the second tab item to synchronize both properties. Moreover, you actually sync those properties on selection change.
In order to achieve your requirement without having the need to manually update your business properties in CodeBehind on selection changed, you should expose the PanOffset and Zoom properties in your main view model. Furthermore, you need to make sure that the RadCartesianChart components are bound to the properties defined in the main view model. By doing so you will ensure that both charting components are bound to the same values of the properties and changing your business properties from either control will update the other one.
I hope this information is helpful.
Pavel R. Pavlov
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