Backup Plans

You can't define schedule by week numbers. Monthly schedule allows you only to run the task on a specific day on a specific week(s) number within a month (1st to 5th), or on specific days of a month (1-31) . If you can use some external script to check the week number, you can use it a a pre-command, schedule both backups to run weekly and fail the backup if pre-command failed (i.e. returned nonzero exit code) , but this will mark the backup job as failed.
You can try to schedule the task to run at 14 days ('since the last completed backup' mode) and also, additionally, on specific days coresponding to mondays of respective weeks. I'm not sure it will work properly, though, as the time is measured since the last completion.