I can make either of them do whatever i want. Most of the time i have the top one on either Radio or CD and the bottom one showing my navigation screen or DVD player. Or vice versa. In the mornings on the way to work sitting in traffic, i watch the news (tv tuner required) on the top one and have the bottom one on navigation or radio. Neither of them do any type of engine or bcm diagnostics or anything like that. I still use my laptop and hptuners sitting on the passenger seat like everyone else for that stuff.
The main reason i did the bottom screen is for passengers in the back and passenger seats to be able to watch DVD’s or whatever while on a road trip while i can still use the navigation on the top screen. Since the top screen is angled directly towards me, it is mainly used just for me since its a touch screen and controls everything. the bottom is facing straight back so its better suited for passengers viewing needs.