I think that you can disagree as long as those two events have « space-like » separation. As you say, they could be events that happened in the past and are in

I think that you can disagree as long as those two events have « space-like » separation. As you say, they could be events that happened in the past and are in your past light-cone. (As long as they happened far enough from each other - or close enough in time.) But one of the events could be your meeting now and here and the other event could be elsewhere. You may disagree on whether that event has happened already or not. Without having any direct confirmation yet, of course. But you may determine later that it was before and after your meeting at t=0.