Hey
I had this problem but have found a solution...
you need to alter the game's config file, found here : C:\Users\USERNAME\AppData\Local\Risen2\Config -> AppData is a hidden folder so can be enabled by going Organise, Folder & Search Options, View Tab and click display hidden files and folders..
In the config file (Open in notepad) there is a section at the top indicating Engine and Window.. In that you need to add the line AdapterNumber="1" (the value for you may be 0 or 2 or 1, sorry will just have to try.
As an example, my config file now looks like this
<options>
<Engine>
<Window
Top="0"
Right="1280"
RefreshRate="60"
Left="0"
FullScreen="true"
Bottom="1024"
AdapterNumber="1"
>
</Window>
<Input>
Hope that helps for you