this.myForm = new FormGroup({
  name: new FormControl('', Validators.minLength(3));
  city: new FormGroup({
    cityOne: new FormControl('', Validators.minLength(3)),
    cityTwo: new FormControl('', Validators.minLength(3))
  }, this.validateEqualCities)
});

validateEqualCities(formGroup: FormGroup) {
    return (control: AbstractControl): { [key: string]: any } => {
    if (formGroup.controls['cityOne'].value &&  formGroup.controls['cityTwo'].value && formGroup.controls['cityOne'].value !== formGroup.controls['cityTwo'].value) {

     formGroup.controls['cityOne'].setErrors({ 'equalCities': true }, { emitEvent: true });
     formGroup.controls['cityTwo'].setErrors({ 'equalCities': true }, { emitEvent: true });

        return { 'equalCities': false };

    } else {
      formGroup.controls['cityOne'].updateValueAndValidity({ onlySelf: true, emitEvent: false });
      formGroup.controls['cityOne'].updateValueAndValidity({ onlySelf: true, emitEvent: false });
    }

    return null;
    };
  }