Nope never tried. I don't understand why you would need to look at both at the same time. I mean, on the DS I never had to look at both screens at the same time. I never thought about it.
When I tested out the 3DS, I was testing the controls, checking out the depth slider, and how good the Dpad feels in your hands, etc. The back and forth in Pilot Wings was because I had to look at the map at the bottom to find my way. It wasn't something I did on purpose. I didn't even think people would have problems switching between screens until it was brought it. It felt really seamless to me.
I was really fatigue when I got my hands on the 3DS demo too, yet I was more than willing to stand around and play with it longer if it hadn't been for the long line of people also wanting to try it out.