I just set up two 24 inch monitors in portrait view connected to an AMD Radeon HD 6800 card. Monitor 1 is connected via VGA cable and DVI adapter. Monitor 2 is connected using an HDMI cable. The resolution on both is set to 1080x1920.
Monitor 1 is fine, but the picture on monitor 2 is shrunk by 2 inches in height and 1 inch in width. A window drug across both screens has a 1 inch drop on the border top and bottom on monitor 2. The font on monitor 2 is shrunk accordingly.