No, there’s no way of doing that. Aside from your specific case of a dual monitor setup, there’s wouldn’t be much application for that; the idea of the screen capture is that the thing eggPlant was looking for wasn’t found, so we show the whole screen so that you can see the state of the SUT and, I suppose, to prove that the image really wasn’t there.
It won’t help with the final screen capture, but you should set the search rectangle at the start of your script to look at just the area (monitor) where your application is running. This will make your scripts run faster because eggPlant won’t have to search the area of the second monitor for every image (which it’s doing multiple times if the image isn’t found immediately.)
set the searchRectangle to (0,0, 1280, 1024)