Gantt Prediction Vs. Actual

such functionality is not supported yet. It’s in a feature list, so it will be available in one of the upcoming version.
As for now, you may try to override task content template, in order to display the estimated task in the same line as the actual. However, it would allow only displaying the ‘estimated’ task, it won’t be draggable separately from the ‘actual’ task object

If you’ll be trying to implement it, you may check this topic, it describes several APIs you will need