Brian will have better wisdom than me. But here's my 2 cents ... even if you get two screens exactly the same, it's very likely that they won't show exactly the same colour, even after calibration. You'll find that one calibrated better than the other. So that one would be your editing screen, and the other one would be your secondary screen, for emails and such.
So because of that, I've never thought it to be entirely necessary to have two matching screens.
However, I'm aware of the attraction of having two matching screens sitting on your desk - it sure does look smart
And strictly speaking, the two monitors don't even need to be the same size. (Mine certainly aren't.) But again - it looks nice if they do