How do you layer a TextView above another TextView inside of a Toolbar in Android?

Question

I'm trying to put one TextView on top of another, but it ends up placing the TextViews beside eachother instead.

Here's my code:

<Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="wrap_content"
    android:id="@+id/toolbar" android:background="?android:attr/colorPrimary"
    android:minHeight="?android:attr/actionBarSize" android:title="@string/blankTitle"
    android:fitsSystemWindows="true">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Upper Text"
        android:layout_gravity="center_horizontal"
        android:id="@+id/title"
        android:textColor="@android:color/white"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Lower Text"
        android:layout_gravity="center_horizontal"
        android:id="@+id/subtitle"
        android:textColor="@android:color/white"
        android:textSize="12sp"/>

    <ImageView android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/queue"
        android:layout_gravity="end"
        android:layout_marginEnd="16dp"/>

</Toolbar>