Yes, all versions of QuarkXPress 7 and above are system independent and can be used on different platforms.
QuarkXPress 6 was for specific platform - either Mac OS or Windows. QuarkXPress 6 developed for one platform will not install on other. Therefore, you cannot use QuarkXPress 6 dual licensing if the platforms are different. Dual licensing on different platform is only possible for QuarkXPress 7 and above.