[andrewflnr]

That's because humans are mostly all in the same reference frame. And hey, so is this binary system, within astronomy- sized error bars anyway. I'm not sure the terminology of "observer" is totally consistent, but it's definitely true that you can only disagree with another observer on ordering of events if you're in a different reference frame, basically if you're moving at a (noticeably) different velocity.

In the presentation of SR I saw, "observers" are basically identical with reference frames. What is "observed" is exactly the sequence of events pieced together from instruments in that reference frame, close enough to "see" all the relevant events with negligible lightspeed delay, all with synchronized clocks. "Seeing" a distant event, the time when light from that event reaches a particular point, is an entirely different question that, frankly, we mostly ignored.