There are two separate MCDU's which may be on different pages at any time. The information they receive is the SAME, but the view not mirrored. In other words, the MCDU's do not mirror one another in the sense that both have to be on the same page. They are "connected" to a single computer, so the data, no matter if put in on left or right side is the same for each. For the record, it feels so wrong saying MCDU for a Boeing product of this generation. That's Airbus and new Boeing verbiage, but I'm just trying to make sure I relay this in a way you understand.