How can a task running twice per day depend on one task running once per day in airflow?

Question

In airflow there are two dag files, a and b, and a dag's c task depends on b dag's d task through the ExternalTaskSensor. a dag is executed twice a day, once at 04:05 and again at 12:05. b dag is executed once a day at 03:00.

How to achieve the above functions? Thanks!