@arborchew,
You can sync toolTips across charts by getting the pixel co-ordinates from the first chart & then triggering an event in second chart and so on. convertValueToPixel() and convertPixelToValue() methods can be used to getting the pixel co-ordinates & corresponding values. Here is a jsfiddle for the same. Please note that there may be a slight variation in x-value as pixel co-ordinates are used.
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Suyash Singh
Team CanvasJS