Normally tasks are updated on the task setup screen one at a time. But if you need to add a task or mutliple tasks to multiple projects at once you can use the multi task update screen shown below. This gives you the ability add or update up to 3 tasks at one time. You can select what projects the tasks go under and what users should have access to these tasks on their timesheet.
When you enter the tasks, the system will determine if the task will be created or updated by matching the task name. If the task name you entered already exists under a project, then the task will be updated. Otherwise if it does not exist then the task will be created.