Tenho shareService e assino que está em outros componentes:

import { Component, Input, OnDestroy } from '@angular/core';
import { MissionService } from './mission.service';
import { Subscription }   from 'rxjs/Subscription';
@Component({
  selector: 'my-astronaut',
  template: `
    <p>
      {{astronaut}}: <strong>{{mission}}</strong>
      <button
        (click)="confirm()"
        [disabled]="!announced || confirmed">
        Confirm
      </button>
    </p>
  `
})
export class AstronautComponent implements OnDestroy{
  @Input() astronaut: string;
  mission = "<no mission announced>";
  confirmed = false;
  announced = false;
  subscription:Subscription;

  constructor(private missionService: MissionService) {
    this.subscription = missionService.missionAnnounced$.subscribe(
      mission => {
        this.mission = mission;
        this.announced = true;
        this.confirmed = false;
    })




  }
  confirm() {
    this.confirmed = true;
    this.missionService.confirmMission(this.astronaut);
  }
  ngOnDestroy(){
    // prevent memory leak when component destroyed
    this.subscription.unsubscribe();

  }
}

Quero saber se tenho 2 assinantes no meuconstructor, como cancelar a inscrição duas vezes em ngDestroy? Eu deveria usarsubscription2:Subscription;? e em ngDestroythis.subscription2.unsubscribe();? isso é verdade?