Looks like the part of the features list that talks about the compatibility between the 3 sets of Photo Studio Lights didn’t make it into the final version of the page. The features list should have this at the bottom:
Fully compatible with Photo Studio and Photo Studio 2 light and presets
All poses and presets for set X1 will work with the light sets from Photo Studio 1
All poses and presets from Photo Studio 1 will work on set X1
All poses and presets for set X2 will work with the Twin Key light sets from Photo Studio 2
All poses and presets from the Twin Key light sets from Photo Studio 2 will work on set X2
All poses and presets for set X3 will work with the Twin Fill light sets from Photo Studio 2
All poses and presets from the Twin Fill light sets from Photo Studio 2 will work on set X3
All of the textures from each of the 3 Photo studio sets will work on each of the 3 background props
Built in DS3.1.2.24