Thanks Priyanka.
I think I need to do it that way because I want the tooltip to appear on the second chart for the same (or very close) timestamp as I mouseover on the first. The pixel location on the first chart is not relevant because of the different y-axis scales of the two charts.
I have worked around the issue for now by padding missing series data with nulls, which gives both charts the same y-axis scale and works well.