How should I configure my tail-lights? I currently own a Cygolite Hotshot which I use with a rack mount in flashing mode. I also have a PDW The red planet which I keep on steady burn.
I wanted something brighter than TRP so I ordered a Niterider Sentinel 40 (which seems to be just like the Solas 40 just with laser lanes), the difference between that and the Hotshot is the diffusing lens on the Sentinel allows for a much larger light spread.

I ride at night on the shoulder (except when I go through intersections, then I'm in the right lane) of a semi-lit intercity 2-lane road where traffic goes at 80-110kph (~50-70mph).

Now I'm not sure which light I should use on steady burn and which in flashing mode. Would it be better for the steady burn to be the focused beam and the flashing to be spread out? Or the other way around?
I run The Bike Light Database (which started from a series of blog posts on this very Stack Exchange site). The Cygolite Hotshot and the NiteRider Solas are actually two of my top recommended lights, and I specifically recommend a setup similar to what you're describing. 

From the recommended taillights page: 


  Putting the Cygolite Hotshot on your rack or stay and the NiteRider Solas on your seatpost is a perfect combo. The Solas has better side-visibility, and on the seat post it can be better seen by traffic on your sides. In flashing mode the Solas serves as the attention-grabber and the Hotshot - which has better straight-on, long-distance brightness - is on steady burn. 


A steady light is easier to judge distance by, so it's better to have the longer-distance light be the steady one. The more attention-grabbing flash is good to have on the wider angled light, so that you are more likely to catch the eye of drivers who are closer to you at an off-angle. 

Here's an image from the page showing one of the recommended configurations, with the Hotshot below on steady and the Solas above flashing.

 

As an aside, I don't personally believe that laser lanes are effective at increasing safety. They seem like a real gimmick to me. Given the extra cost of the Sentinel taillight, you might be better off returning it (if you can) and just getting the regular Solas taillight, and then spending the money you saved on something different.