...and nobody here is likely to be any more sure than you Muz - because it's not a normal thing to do.
Presumably you want 2-channel stereo outside. In which case you'd need a switcbox to switch between your internal (main) and external pairs - after selecting 2-channel stereo mode (if your amp has that?).
Whether or not the amp would allow you to mute the unwanted indoor speakers, I don't know.
I do know that I would find another way to get outdoor sound if I was you 👍