Are the scientists interacting? What is the barrier you see? I think the two systems interact simply means that any event you can measure is one you have interacted with (however indirectly). It isn't a limitation on the number of systems interacting with each other.
> "If a tree falls in a forest and no one is around to hear it." I guess RQM would say that this question isn't even wrong.
I think the more concrete interpretation is being used. To "hear" something, you have interacted with it in some way.
Not hearing something, in any way, simply means you have not yet interacted with it. There has been no exchange of information. It doesn't mean it didn't exist.
The point that there is no privileged system or observer underlines that existence isn't tied to "observers".
If both we and another civilization started sending radio signals into space on the same date, and on another future date both civilizations finally got the first signals from the other, neither detection caused the other civilization to come into existence. They both existed separately before the common detection. There is no privileged observer from whom to relate what exists or not by their observations.
Yes, trees that fall in a forest without human beings cause vibrations. They interact with the forest around them. There is no observer-tron particle in humans that acts in a special way on events around us.
But between a photon being emitted and absorbed (two bracketing events on its existence), we might say the photon's path is just those interactions, and there isn't anything to talk about between them. If it had no interactions with anything else between its two bracketing events.
Like a discrete edge in a simple graph, it is a connection between two nodes, there are no dynamics in between the nodes. An edge is just a connection, not a continuous path.