Simultaneity Relations Relative to Multiple Observers

I challenge the assumption in the debate on the conventionality of simultaneity that a simultaneity relation of special relativity should be defined relative to a single inertial observer, not relative to multiple inertial observers as such. I construct an example of a simultaneity relation relative to two inertial observers, and demonstrate that it is explicitly definable in terms of the causal connectibility relation and the world lines of the two observers. I argue that, consequently, thesimultaneity relation of special relativity is not uniquely definable from the causal connectibility relation