If the monitors work properly, then this could be either a cabling issue, driver issue, or hardware issue. Best you can do is uninstall/reinstall the video drivers. Try using a different 'output' , such as VGA to see if it's a prob with the DVI port/cable. Also, are you able to reverse the two? (depending on the inputs of both monitors). My suspect is some sort of hardware issue. Has it ever worked? If so, when did it start, can you maybe run a system restore to that point?
Something to start with, sorry a little dry on answers for this one.