android: how todivide linear layout elements equally

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" 
    android:weightSum="2">

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:enabled="false"
        android:text="Installed engines" >
    </Button>

    <ListView
        android:id="@+id/primary"
        android:layout_width="match_parent"
        android:layout_height="0dp" 
        android:layout_weight="1" >
    </ListView>

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:enabled="false"
        android:text="Active engines" >
    </Button>

    <ListView
        android:id="@+id/secondary"
        android:layout_width="match_parent"
        android:layout_height="0dp" 
        android:layout_weight="1" >
    </ListView>

</LinearLayout>