How to do the Flattening in Rxjs for Angular Auth Service?

I have created a Authentication service in Angular with function SignUp that sends the API Request to Firebase, As Firebase returns the User ID, I am saving the userid into my personal MongoDB Database with its Role. Now the problem here is i am sending two request which i want to further Subscribed in Register.component.ts, I am not able to understand how to achieve this. Below are the sample code that i have tried.

auth.service.ts

 signUp(email: string, password: string) {

return this.http.post<AuthResponse>(`https://identitytoolkit.googleapis.com/v1/accounts:signUp?key=${config.API_KEY}`, {
  email: email,
  password: password,
  returnSecureToken: true
}).pipe(
  switchMap(data => { 

    return  this.http.post<any>(`${config.BASE_URL}/api/ezusers/`,{useruid: data.idToken, 'isRegistered':false}); // or this.userId
  })
).map(response => {
this.authenticatedUser(response.email, response.localId, response.idToken, +response.expiresIn);
  // this.userOrders = response;
  return
});

}

Register.component.ts

onSubmit() {
this.loading = true;
if (this.registerForm.valid) {
  this._authService.signUp(this.registerForm.value.email, this.registerForm.value.password).subscribe(
    res => {
      console.log(res);
      this.loading = false;
      this.registerForm.reset();
      this.success = true;
      this.error = false;

    },
    err => {
      console.log(err);
      this.loading = false;
      this.success = false;
      this.error = this.errMsgs[err.error.error.message];

    })



}
else {

}

}

Any help would be really Appreciated.

Thanks in Advance!