I think it helps to have played the game at Expo with the full screen option up top. It actually makes a lot of sense to be mirrored already. And on Alien I'm reworking the interface design now to work with the strengths of both screens to make it even better now that I've seen how Full Throttle works, and thinking about what I might change.
Here's how I'm approaching the user interface of the LCD screen:
You should see your score pretty much all the time. The exception being full screen animation takeovers. Like say the start of a multiball. But then you're back to seeing your score.
That works great for both screens. I want that info on both screens.
And during modes I want the status of the mode completion in both places. This isn't The Hobbit, they have a more "dashboard" approach. The Alien screens are going to be more like a traditional DMD, but with a higher level of detail, in full color, using clips from the film. So you can look to either place for vital information while playing, whichever you prefer. I think you're going to look up a lot more than you think if you install a second screen up there.
Anyways, long way of saying that after playing it myself at Expo, and thinking about how it should work, I think mirroring it is the way to go, but with a design built to run on both from the start. It will honestly provide the best playing experience.
I'm actually pretty excited about making something that works across both. It's a great design challenge, because it forces you to really attack the UX from multiple use cases, and as a result you're going to have a design that works at different scales and angles. Lots of chances for iconography for instance.
Part of what I do for a living is design user interfaces, it's a good challenge, and I think the right approach.