I think, but I'm not sure, the only way to do that is to move the LR main screen to that second monitor. The before and after view always happen in the main LR screen.
Thanks for the reply, this is what I am doing as a work around. Wish it was possible to move the side panels to the 2nd monitor this would fix the issue.
If you are looking for more space to view the before and after image you can turn the side panels off. Just hit the Tab key and they hide. Hit it again and they show.