As you are using Angular Reactive forms, you can listen for changes in the dateOne form control value through the valueChanges observable and set the dateTwo form control value.

this.form.get('dateOne')?.valueChanges.subscribe(date => {
  const dateTwo = formatDate(new Date()+ 7, 'MM/dd/yyyy', 'en');
  this.form.get('dateTwo')?.setValue(dateTwo);
});