I don't know the international policy, but in the Netherlands I called Nuance about installing on more than one system I was allowed to install on two different computers if the copies wouldn't be used at the same time, because it's a single user license. You should find out if you really need to buy another copy.
I would be willing to buy a second copy to be used concurrently on both computers, but I am concerned that DNS-11 may not operate with all features as well under Linux as it does under Windows. Does anybody in the Linux world have experience with DNS-11, yet?