Angular 5 ng-select how to add two values to ‘bindLabel’?

It is possible to display it via a custom label and item template:

<ng-select [items]="users" bindLabel="firstName"> 

  <ng-template ng-label-tmp let-item="item">
      <span >{{ item.firstName + ' ' + item.lastName }}</span>
  </ng-template>
  <ng-template ng-option-tmp let-item="item" let-search="searchTerm" let-index="index">
        <span >{{ item.firstName + ' ' + item.lastName }}</span>
  </ng-template>

</ng-select>

ng-select only accepts a string value in the attribute. I may be misunderstanding but I believe that if you say bindLabel="firstName+lastName", ng-select is attempting to reference item[firstNamelastName] which does not exist.

I think your best option is to transform the collection.
You can add a .map to the end of your array declaration and use bindLabel="fullName" in your template:

[
  {firstName: "John", lastName: "Doe"},
  {firstName: "Jane", lastName: "Doe"}
].map((i) => { i.fullName = i.firstName + ' ' + i.lastName; return i; });

<ng-select [items]="users" bindLabel="firstName"> 
    <ng-template ng-label-tmp let-item="item" let-clear="clear">
        <span class="ng-value-label">{{item.firstName + ' ' + item.lastName}}</span>
        <span class="ng-value-icon right" (click)="clear(item)">×</span>
    </ng-template>
</ng-select>