What title says. There are often two parallel activities, but I want of them to be on top of the other. Is it achievable?
If there are two tasks on your timeline view that have the same date, then their vertical order is determined by their order in Spreadsheet View. You can reorder them in this view by drag and drop to achieve the desired vertical order on the timeline view.
This is only possible for events with the same date, otherwise the vertical order is determined by the layout function chosen and its date.