The Tooltip and the Crosshair are linked and so the Tooltip will only be shown when the Crosshair is shown. Furthermore, the only way to show the Crosshair is by performing a long touch gesture. As such it's not possible to show the Crosshair with a single touch gesture (a single touch of course being used to remove the Crosshair when it is active).
You could of course add your own View on top of the ShinobiChart and use the ShinobiChart.OnGestureListener to be notified of any single touches on the chart (showing and hiding the View as necessary). While you can then make use of the Axis.getUserValueForPixelValue() method to convert from the touch point in pixels to values in terms of your data the main difficulty would be finding the closest data point in your series based on the touch position.
I'm sorry I can't be of any further help with this.