Hi there - I can see a request for information from you that came through from our support person back in November last year but I don't know if they replied and tried troubleshooting, we're not seeing any replies to you in their mail archives.
This isn't a general problem we have seen, most people are able to use ArtRage 5 with dual monitors without seeing the problem you are seeing, so we will need to do some troubleshooting to find out what is causing the problem in your case. If you could mail me at
[email protected] and let me know the hardware specifics I'll get some steps together and we can work on fixing this for you!