Some users need to have both the Start Date and End Date of a task to be controlled by other tasks. This can be done in Time Line by creating three tasks: two 'dummy' details indented below one summary (the 'hammock' task).
The two indented details should both be milestone tasks (0 duration), with the first detail task ASAP and the second detail task ALAP. The task that controls the Start Date should be connected as a predecessor to the first detail, and the task that controls the End Date should be connected as a successor to the second detail.
The 'dummy' details can be hidden by collapsing the summary task. In this example, resources and costs can be assigned to the summary task, as this is where the total duration is calculated. However, keep in mind that Time Line only levels details tasks, and will NOT resource level summary tasks.