Windows that detach from the main Spine window are more difficult to create for the technology Spine uses to run using OpenGL on Windows, Mac, and Linux. We do have plans to have floating windows within the main Spine window, so you can position any view as either a tabbed pane or floating window. This will happen before detached windows, which are unfortunately not planned in the short future.
As a workaround, you can size your Spine window so it spans multiple monitors. Here are a couple tips when doing that.