Those panels from different lots have somewhat different colors anyway, even in the same orientation, and you can't tune them to be equal very easily, at least I gave up. They have pretty good viewing angles too. So it doesn't make it better or worse to turn either one 90 degrees.
Did you use a hardware calibrator? It shouldn't be hard to get a Dell monitor to a calibrated white point - they're not bad unless you got a TN panel model.
