I'd like to investigate how the notion of "mutual rest" might be applied consistently, but distinctively, in the following thought experiment:
Consider a light source ("$A$") which directs a beam towards a receiver ("$B$"). Source $A$ is modulating the beam, emitting discrete recognizable signals in rapid succession (i.e. with short durations from stating one particular recognizable signal modulation until stating the next; compared to $A$'s duration from stating one particular recognizable signal modulation until observing the corresponding echo from receiver $B$).
Is there some definite, operational sense of "mutual rest" whereby in this case it could be said
that source $A$ and receiver $B$ had been "at rest to each other" ("throughout the experiment", if applicable); but
that a successive pair of signal fronts had not been "at rest to each other" ("while both were on their way")