RolloverModifier - show one tooltip when having multiple datasets

There is no dedicated property for that, you’ll need to override the modifier.
The main point to override is CalculateTooltipPositions method. It recieves enough info to perform all of the necessary calcaulations. (i.e. find the nearest data point)
There the required info could be extracted from the results of hit test.

Depending on the requirements of behaviour on “pointerleave” and “pointerout” events, you might need to override the corresponding handlers as well. See Custom Chart Modifiers API

Full live example at CodeSandbox

class NearestPointRolloverModifier extends RolloverModifier {
  protected CalculateTooltipPositions(
    tooltipArray: TTooltipProps[],
    allowTooltipOverlapping: boolean,
    spacing: number,
    seriesViewRect: Rect,
    pixelRatio: number,
    isVerticalChart: boolean = false
  ): TTooltipProps[] {
    const allTooltips = super.CalculateTooltipPositions(
      tooltipArray,
      true,
      spacing,
      seriesViewRect,
      pixelRatio,
      isVerticalChart
    );

    let min = Number.MAX_SAFE_INTEGER;
    let nearestTooltip: TTooltipProps = null;
    allTooltips.forEach((tooltip) => {
      if (tooltip.seriesInfo.distance < min) {
        min = tooltip.seriesInfo.distance;
        nearestTooltip = tooltip;
      }
    });

    return [nearestTooltip];
  }
}

 // usage
  sciChartSurface.chartModifiers.add(
    new NearestPointRolloverModifier({
      rolloverLineStroke: appTheme.VividOrange,
      showTooltip: true,
      hitTestRadius: 100000, // some large number to apply hit test detection on the whole viewport
    })
  );