By default the main scene creator file contains 1 of each device - MacBook, iMac, iPad and iPhone. If you wish to duplicate a device, for example have 2 iPads then please follow the steps below.
Make sure the main folder group is selected in the layers panel (in this example 'Portrait iPad' group. Right click on this group and select 'Duplicate Group'.
The duplicated group will appear on top so move it to one side so that you can see both of the devices.
You will notice that the designs in both the iPads are the same. To show different designs follow these steps:
Expand the 'Portrait iPad copy' group. Right click on the 'Add Design Here' layer highlighted red and select 'New Smart Object via Copy'
You will now have 2 'Add Design Here' smart layers
You can delete the layer you duplicated from, leaving only the copied version.
You will now need to make sure that this new copied layer is 'clipped' to the layer below. Simply right click on the new duplicated layer and select 'Create Clipping Mask'.
You can tell that the layer is 'Clipped' to the layer below by the new layer being indented and now has a small down arrow.
To add your design to this new layer simply follow the steps in the 'How To Add Your Designs' article.
Once you have added your design to this new iPad you will see that they now have different designs. If you wish to have a third iPad then simply duplicate the group of one of them and follow the steps above.
As an update to this scene creator, there are now 'pre-made scenes' which contain various layouts of different devices which make it very easy to showcase multiple devices with different designs.