Manually? As mentioned by Anupam, you should be able to use the repair function available on the jobs UI. You can also specify different parameters each time you repair specific tasks to be more precise about the datasets you want to process.

If you wish to trigger it programatically, then it depends. Here are a couple scenarios.

If the couple tasks need to be triggered frequently, it may make sense to contain them within a new job.

If they need to be triggered due to failure within the multi task job you can look into specifying retries within the tasks themselves.