Quote - I reported this same problem to SM, and their answer was not to stretch the app window across both monitors. (I used to press shift-maximise to do this)

Instead I have posewindow docked on the main screen and all the other stuff floating.
Works great.

Yup, same here. There's no need to expand the whole Poser window across both screens, when you can float off some of the windows to your second screen. Won't your method give you some docked windows spanning the gap between screens?

But if you really want it that way, it looks like you need a bug fix.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)