Wrapping each horizontal LinearLayout in a FrameLayout achieved the desired effect.
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:orientation="vertical" >
<FrameLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.25">
<LinearLayout
android:id="@+id/linear1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="horizontal" >
<ImageButton
android:id="@+id/imageButton2"
android:layout_width="0dp"
android:layout_weight="0.40"
android:layout_height="fill_parent"
android:src="@drawable/ic_pages" />
<ImageButton
android:id="@+id/imageButton1"
android:layout_width="0dp"
android:layout_weight="0.60"
android:layout_height="fill_parent"
android:src="@drawable/ic_pages" />
</LinearLayout>
</FrameLayout>
</LinearLayout>
</RelativeLayout>