you can have multiple windows open at once and switch between them quite freely. just press back or select on your controller and it flips the context between open windows. i do this frequently when swithcing jobs and going thorugh my armory chest to remove stuff I no longer use.

opening multiple windows though yeah thats a bit of a pain. you need to hot bar them all or create keybinds..
but what you can do to save space is put them on a cross hotbar you never use such as 8. and then can press r1 + left dpad to jump to that croos hot bar (even if its disabled) when you want to open menus.