Joomla MVC works based on task parameter, following a CMD string pattern. In this way, it can be associated with a controller/task.
A task array is not standard parameter, and Joomla MVC is not going to accept it, or call several tasks to compose a final output.
Tasks are executed by JControllerLegacy in this way:
public function execute($task)
$this->task = $task;
$task = strtolower($task);
$doTask = $this->taskMap[$task];
$doTask = $this->taskMap['__default'];
throw new Exception(JText::sprintf('JLIB_APPLICATION_ERROR_TASK_NOT_FOUND', $task), 404);
// Record the actual task being fired
$this->doTask = $doTask;
In theory, you can override all JController methods to override the default task execution, and create your own processing. Or, you can just add a new parameter mytask, and manage mytask in your own way to avoid any conflict.