android:layout_height 50% of the screen size

Set its layout_height="0dp"*, add a blank View beneath it (or blank ImageView or just a FrameLayout) with a layout_height also equal to 0dp, and set both Views to have a layout_weight="1"

This will stretch each View equally as it fills the screen. Since both have the same weight, each will take 50% of the screen.

*See adamp's comment for why that works and other really helpful tidbits.

This is easy to do in xml. Set your top container to be a LinearLayout and set the orientation attribute as you wish. Then inside of that place two linearlayouts that both have "fill parent" on width and height. Finally, set the weigth attribute of those two linearlayouts to 1.

This is my android:layout_height=50% activity:

<?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:id="@+id/alipay_login"
        style="@style/loginType"
        android:background="#27b" >
    </LinearLayout>

    <LinearLayout
        android:id="@+id/taobao_login"
        style="@style/loginType"
        android:background="#ed6d00" >
    </LinearLayout>

</LinearLayout>

style:

<style name="loginType">
    <item name="android:layout_width">match_parent</item>
    <item name="android:layout_height">match_parent</item>
    <item name="android:layout_weight">0.5</item>
    <item name="android:orientation">vertical</item>
</style>