There is no way to do that now. But, if you want to add settings for it,
please feel free to commit changes.
I would like to avoid settings as much as possible... it should be
automatic (e.g. resolution close to 32:10 is probably a dual monitor).
In the end I modified directly GTSpotterGlobalShortcut
class>>#openGlobalSpotter, however it's not generic (so I am not going to
commit it yet).
As I found, there are more things that do not play well with dragging Pharo
over two monitors: (GT)Debugger, taskbar, growl, ... and possibly many
others, so maybe some dual-monitor care package will emerge as I will fix
the issues one by one.