this.form = fb.group({
  name: ['', Validators.required],
  email: ['', Validators.required]
  matchingPassword: fb.group({
    password: ['', Validators.required],
    confirmPassword: ['', Validators.required]
  }, {validator: this.areEqual})  <--------
});

this.form = fb.group({
  name: ['', Validators.required],
  email: ['', Validators.required]
  matchingPassword: fb.group({
    password: ['', Validators.required],
    confirmPassword: ['', Validators.required]
  }, {validator: MYCustomValidators.areEqual })  <--------
});

import {Directive, forwardRef} from '@angular/core'
import {NG_VALIDATORS, FormControl} from '@angular/forms'

function validateDomainFactory() {
  return (c: FormControl) => {
    let VALIDATION_REGEXP = /^([a-zA-Z0-9][a-zA-Z0-9-]+\.)?[a-zA-Z0-9][a-zA-Z0-9-]{0,61}[a-zA-Z0-9]\.[a-zA-Z]{2,}$/i

    return VALIDATION_REGEXP.test(c.value) ? null : {
      validateDomain: {
        valid: false
      }
    }
  }
}

@Directive({
  selector: '[validateDomain][ngModel],[validateDomain][formControl]',
  providers: [
    { provide: NG_VALIDATORS, useExisting: forwardRef(() => DomainNameValidator), multi: true }
  ]
})
export class DomainNameValidator {
@Input() secondFiled: FormControl // <<<<--- сюда передаем второй контол от которого зависим
  validator: Function

  constructor(/*Dependency Injection here*/) {
    this.validator = validateDomainFactory()
  }

  validate(c: FormControl) {
    return this.validator(c)
  }
}