No Video Output on Second Screen During Startup?
The 'one screen at boot' is pretty standard behaviour, afaik. I see it on all the Mac Pros here. 

Which screen that ought to be is dependant on which is set as Primary in System Prefs > Displays > Arrangement - it's the one with the menu bar, which can be dragged to another display to set Primary.



That it still displays the same behaviour with one screen switched off is because it still recognises it as being attached. When you detach it completely, then basically you get an "Oops, Primary is missing, let's find one we can use temporarily."