I have this setup, with a 20" wide monitor in portrait mode, and a 23" wide in landscape mode. I tried a 23" in portraitmode as well, but found it to be to high for comfortable viewing angel.
IF you decide to go for a portrait monitor, make sure you choose one with good viewing angels (an IPS panel preferably), because most monitor will dramatically lose color and screw up the contrast when viewed from down below (from the left in portrait on 90 degrees clockwise rotation).
You might even suffer from color bleeding on the pixels if you have a bad panel, so be sure to test it thoroughly before purchasing your screens.
I really like it a lot, when programming and logfile reviewing a portrait mode is excellent. As an example, I have almost 100 lines of code visible at the same time in visual studio, and equally the same in my text editor. Even on my fullHD 23" I only get around 60 rows so that is approx 65% more lines.