Android nested LinearLayouts

gistfile1.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MyActivity">

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal"
        android:weightSum="2">

        <ImageButton
            android:id="@+id/taskButton"
            android:layout_width="0dp"
            android:layout_height="fill_parent"
            android:layout_weight="1"
            android:onClick="onClick"/>

        <ImageButton
            android:id="@+id/phoneButton"
            android:layout_width="0dp"
            android:layout_height="fill_parent"
            android:layout_weight="1"
            android:onClick="onClick"/>
    </LinearLayout>

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:orientation="horizontal"
        android:weightSum="2">

        <ImageButton
            android:id="@+id/musicButton"
            android:layout_width="0dp"
            android:layout_height="fill_parent"
            android:layout_weight="1"
            android:onClick="onClick" />

        <ImageButton
            android:id="@+id/calendarButton"
            android:layout_width="0dp"
            android:layout_height="fill_parent"
            android:layout_weight="1"
            android:onClick="onClick"/>
    </LinearLayout>
</LinearLayout>