as it is now, on linux you can open multiple scribus in parallel.
it might be possible to add a command line option to open the documents in the same instance (in the past gimp used to have a separate executable for that).
i don't know if it's possible to make sure that the launching the same executable uses a common instance but the same version from different executables use separate instances.
if it's doable, it's something that a "normal" skilled programmer should be able to implement. but there are chance that different code is needed for unix, windows and macos.