Align two-button layout left and right in Android

Use a RelativeLayout. There you can set android:layout_alignParentLeft and android:layout_alignParentRight.
This should work for you:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    android:background="@android:color/white"
    android:gravity="bottom" >


    <RelativeLayout
        android:id="@+id/relativeLayout1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="@android:color/black" >

        <Button
            android:id="@+id/button1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="prev"
            android:layout_alignParentLeft="true" />

        <Button
            android:id="@+id/button2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="next"
            android:layout_alignParentRight="true"/>

    </RelativeLayout>

</LinearLayout>