I haven't done it starting both jobs at the start, but I started a national job once then maybe a week or so picked up a vacant club team.. Not sure if that helps???
- Start game as usual with club of choice.
- Make a 2nd Manager as manager of the National Team you want.
- Use 2nd manager to appoint yourself as U21 NT manager
- Retire 2nd manager.
- Declare interest in main NT job.
- You should get the job.