I have one Samsung 213T and don't have room for another on my desk (let alone the money to purchase an additional $650 monitor).
Depending on what you want to do, identical may or may not be necessary. For example, when working in Photoshop, a second monitor may be used to keep all of the palettes out of the main working area; in this case, the second monitor can be a smaller 15 or 17 inch model.
What would be most important to me is that the pixel pitch of the monitors would be matched. Physical size differences are fine, but having a window change in size when moving it from one monitor to the next is highly annoying IMHO.