How to half overlap images in android constraint layout

Simplest way

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    >

    <ImageView
        android:id="@+id/appBar"
        android:layout_width="match_parent"
        android:layout_height="300dp"
        android:background="#c2b6c2">

    </ImageView>


    <android.support.design.widget.FloatingActionButton
        android:id="@+id/floatbtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:src="@drawable/ic_menu_camera"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintBottom_toBottomOf="@+id/appBar"
        app:layout_constraintTop_toBottomOf="@+id/appBar"
         />
</android.support.constraint.ConstraintLayout>

you can set layout using only constraint layout like below :

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/imageView4"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@android:color/holo_orange_dark"
        app:layout_constraintBottom_toTopOf="@id/guideline"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0" />

    <android.support.constraint.Guideline
        android:id="@+id/guideline"
        android:layout_width="0dp"  
        android:layout_height="0dp"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.5" />

    <ImageView
        android:id="@+id/imageView_upper"
        android:layout_width="70dp"
        android:layout_height="70dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="@id/guideline"
        app:layout_constraintBottom_toBottomOf="@id/guideline"
        android:background="@android:color/holo_purple"/>

</android.support.constraint.ConstraintLayout>

Note: If you are using androidx then you have to use androidx.constraintlayout.widget.ConstraintLayout instead of android.support.constraint.ConstraintLayout