FlightGear does support multiple windows - however, the current GUI cannot make use of that feature.
The Canvas system could relatively easily support placements in separate Windows, and that is in fact something that was originally planned.
Furthermore, there's currently ongoing work to implement/improve CompositeViewer support, which is basically about multi-window/multi-viewer setups:
https://wiki.flightgear.org/CompositeViewer_SupportAgain, in its current form, this does not include support for the GUI (PUI) - however, a dedicated Canvas placement for "pop-out" windows is indeed a recurring feature request:
https://wiki.flightgear.org/Canvas_Popout_WindowsIn particular, that would also mean that Canvas based avionics (like the FG1000) could not just be shown inside the cockpit or in Canvas GUI dialogs, but also inside dedicated OSG windows, e.g. to display such avionics on dedicated screens, possibly even using a dedicated GPU:
https://wiki.flightgear.org/FG1000If in doubt, please file a proper feature request here:
https://sourceforge.net/p/flightgear/codetickets/