WAB does not have a side by side map theme. So you would have to do a lot of custom development to achieve this. AGOL does have to templates for this though:
You can do this by creating a custom widget.
In this widget's setting page, you can config the map that you want to show side with the main map. In the widget, you can access the main map and the map configured for this widget, so you can sync the state of the 2 maps, like sync extent.
For the side by side view, you can use the dashboard theme to create a layout and then add the widget to show side with the map.