Yes, on my PC running the Frozzbos Extreme Edition OS it works very well 😀

what is "frozzbos Extreme edition OS"?
and just to clarify im using windows xp Sp2
And i have tried all that i can think of to get dosbox to goto the second screen with no luck

Some operating system I made up since you didn't specify which OS you wanted to run DOSBox under. So maybe it does not work with WinXP. Or maybe it is a limitation in your graphics driver. As long as you don't tell us any details of your setup all we can do is to guess. There is a reason why I have the link to "How to ask questions the smart way" in my signature. You should read http://www.catb.org/~esr/faqs/smart-questions.html#beprecise

Maybe some SDL environment variable would specify the target
of the output, but no real idea.

AFAIR, good ol' MS-DOS never supported dual screen display, so it's probably way out of the scope of this project... I mean, DOSBox is meant to capture that "warm feeling" of playing games from the golden days of DOS, and unless this new "feature" could be easily applied by comunicating with the OS display driver, it would really be a whole lotta work for a very little (if any) reward.

No, think he doesn't mean splitscreen support or something,
but displaying the regular content of the dosbox window
(additionally/exclusively) on the second screen.

Oh... ok! I misunderstood then.

However, I think such a function is available on specific graphic cards and drivers. Might be mistaken though... Isn't it possible through driver selection to assign diferent displays/outputs to 2 monitors?

Anyway, it's probably more troublesome than the final reward itself, but interesting concept, nonetheless! 😎