I mostly shoot indoors, so what is the advantage of HSS?
You have full control of the ambient light if you, for example, shoot close to windows. With HSS you can control all ambient light and be in full control of the lighting. There is no need to use ND filters on the camera to control the ambient lighting. If you shoot some type of action in mixed light, it is great to use HSS to freeze the movement. Using a regular flash may frequently result in motion blur since the sync is normally not faster than 1/250s.
Is there any advantage to choose the Duo Kit with 2 heads, the price is the same as two single units?
Yes, you get the new Bag S Plus, a sturdy premium bag. The Bag S Plus can also fit D1/D2/B1 units with the optional glass dome.
Can I change the orientation of the display?
Yes, select the special setting menu by pressing the settings button for 3 seconds to display a setting for display orientation.
I normally travel to various places in the world, does the D2 work globally or do I need a special version?
Both the D1 and the D2 are multi-voltage. That means you can use them anywhere around the globe. Just plug and play!