But if there is no meaningful measurement of them in isolation, how are they a meaningful unit?
Still - lets look at interdependency.
Is the lock dependent on his other lock at scrum time? No more than the prop in front of him, with the flanker besides him and the hooker in front of him also quite important.
Is the lock dependent on his other lock at the line out? He is most dependent on the hooker and the two men lifting him. The latter may include the other lock, particularly in a short lineout, but its unlikely. We're looking at props in particular and the back row as well. He's a bit dependent on the other jumpers to distract the opposition, but there's no guarantee the other jumper they're most fixated on is a lock.
Is the lock dependent on his other lock when breaking from a set piece? Piece of string. Maybe they hit the same ruck. Maybe one player is lining up as the next carrier while the other clears the ruck with a centre. Maybe one lock clears the ruck while the other lurks on the blindside. That's assuming a scrum. If its a maul then, well, anyone could emerge first. If its from a lineout, its not unusual to see the jumper stay out on that wing as the player sweeps to the other side, at which point its not unusual to see the locks with most of the pitch between them.
I really don't see a huge amount of inter-dependence there. Maybe the locks are more inter-dependent, but not to any meaningful degree.