I find the reason it's hard to do both on the same project is that the engineer by definition knows the internals of the system. It's hard to un-know that and see things afresh from the user perspective. Thus one tends to create UIs that are intuitive to someone who already has a mental model of how the system works, but seem convoluted and arbitrary to everybody else.
This is a good idea. I'm going to try to do it on my next project. I don't know if I can stop thinking about the data and implementation. It is just where my mind naturally goes. Someone presents me with some type of data and a way they want to present it and I immediately start thinking about data representation, persistent storage, data abstraction models.
I think more important is just to get out of the engineering mindset and try to think like a user. I bet just sitting down with a non-technical person and working through the UI design would help a lot to keep me from going into my engineering mindset.
