<Button
     android:id="@+id/button_tag"
     android:layout_width="match_parent"
     android:layout_height="40dp"
     android:layout_marginTop="1dp"
     android:background="@drawable/img_whitebackground"
     android:gravity="left|center_vertical"
     android:paddingLeft="5dip"
     android:text="@string/string_tag"
     android:onClick="click_addscreen" >
 </Button>

 <Button
    android:id="@+id/button_rate"
    android:layout_marginTop="1dp"
    android:layout_width="match_parent"
    android:layout_height="40dp"
    android:text="@string/string_rate"
    android:gravity="left|center_vertical"
    android:paddingLeft="5dip"
    android:onClick="click_addscreen"
    android:background="@drawable/img_whitebackground" >
</Button>

RelativeLayout.LayoutParams p = new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
                    ViewGroup.LayoutParams.MATCH_PARENT);

p.addRule(RelativeLayout.BELOW, R.id.button_rate);
buttonTag.setLayoutParams(p);