All you need to do is define a group
with an unique ID, I have checked the implementation if group has different id’s it will create a divider.
Example menu, creating the separator:
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
tools:context=".MainActivity">
<group android:id="@+id/grp1" android:checkableBehavior="single" >
<item
android:id="@+id/navigation_item_1"
android:checked="true"
android:icon="@drawable/ic_home"
android:title="@string/navigation_item_1" />
</group>
<group android:id="@+id/grp2" android:checkableBehavior="single" >
<item
android:id="@+id/navigation_item_2"
android:icon="@drawable/ic_home"
android:title="@string/navigation_item_2" />
</group>
</menu>