Sorry, I got bored and lost interest. What is it you wish to know? I thought we answered the OP. How can we see crux? Because it is part of the celestial gearing system that revolves above us. It can't be seen at opposite sides of the earth at the same time because it is day.
Its also not possible on a round earth because stars rise and set. the two places you picked would experience rising setting problems preventing you seeing them anyway.
The OP is actually a waste of time. You claimed two arbitrary points could both see a constellation at once. You had no source data for either. You made an assumption you should be able to see the constellation, (actually a poorly based assumption round earth or flat) and then said 'well it must be like that so how can it be replicated on a flat earth?'.
Its a nonsense thought experiment. Unless you return with reliable observational data, there really isn't very much to talk about. This thread is just the misconceptions in your head. Hence the reason I lost interest.