The method allows you to broadcast both elements simultaneously to avoid any latency issues with the mobile stream.
To accomplish this you will be using the Screen Share plug in for Open Broadcaster and a camera video feed source.
Additional elements can also be applied to your live stream such as a banner or a text field.
Settings I used for the Screen Share was 1920 X 1080.
This can be adjusted to fit your needs.
The second capture used was a HD camera with the overlay settings at 32X176 and opacity set at 60% to allow the slide images to be viewed through my live stream.
The overall bit rate for the stream is set at 996Kbps and with the audio settings set to AAC to allow your stream to be viewed on all devices.
This image shows the multiple sources that can be used with Open Broadcaster. You can also switch between the sources during your live stream.
For getting started with Open Broadcaster click here.