Использование LinearLayouts и установка детей на fill_parent как по ширине, так и по высоте и присвоение им одного и того же layout_weight вы получите желаемый эффект:
<?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">
<LinearLayout
android:orientation="horizontal"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<Button
android:text="Button 1"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"></Button>
<Button
android:text="Button 2"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"></Button>
<Button
android:text="Button 3"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"></Button>
</LinearLayout>
<LinearLayout
android:orientation="horizontal"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<Button
android:text="Button 4"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"></Button>
<Button
android:text="Button 5"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"></Button>
<Button
android:text="Button 6"
android:layout_weight="1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"></Button>
</LinearLayout>
</LinearLayout>
awesome thank you =) –