Preview layout with merge root tag in Intellij IDEA/Android Studio

There is a new parentTag tools attribute (added in Android Studio 2.2) that you can use to specify the layout type for a merge tag, which will make the layout render correctly in the layout editor preview.

So using your example:

<merge xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:parentTag="LinearLayout"
    tools:orientation="horizontal">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Some text"
        android:textSize="20sp"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Some other text"/>
</merge>

Note: Both android:layout_width and android:layout_height must be specified in order for the layout to display properly in the editor.

Edit: Outdated answer. See answer by starkej2.

---

Android Studio 0.5.8 added support for tools:showIn. By using it it is possible to preview < merge > layouts.

http://tools.android.com/recent/androidstudio058released

layout/layout_merge.xml with tools:showIn:

<merge xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:custom="http://schemas.android.com/apk/res-auto"
   xmlns:tools="http://schemas.android.com/tools"
   tools:showIn="@layout/simple_relativelayout">

......

</merge>

layout/simple_relativelayout.xml with include:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <include layout="@layout/layout_merge"/>

</RelativeLayout>