ng-bootstrap-dropdown-split.html

<div class="btn-group mr-3">
  <button type="button" class="btn btn-primary">Split me</button>
  <div class="btn-group" ngbDropdown role="group" aria-label="Button group with nested dropdown">
    <button class="btn btn-primary dropdown-toggle-split" ngbDropdownToggle></button>
    <div class="dropdown-menu" ngbDropdownMenu>
      <button ngbDropdownItem>One</button>
      <button ngbDropdownItem>Two</button>
      <button ngbDropdownItem>Four!</button>
    </div>
  </div>
</div>