Consider 1+1 Minkowski spacetime.

Consider two events A and Z which are not causally related.

Their future light-cones intersect at an event (call it F) and
their past-light cones intersect at an event (call it P).

https://www.desmos.com/calculator/mveyzcilol

The inertial observer Bob through P and F observes that A and Z are simultaneous since Bob performs a "radar experiment" to assign a time-coordinate to event A by sending (when Bob's clock reads $t_P$) a signal to A and waiting for the arrival of its echo (when Bob's clock reads $t_F$). Bob assigns a time-coordinate to A using his clock readings $t_P$ and $t_F$: $$t_{A, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2}.$$ Similarly, $$t_{Z, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2},$$ which is equal to $t_{A, \rm accd\ to\ Bob}$.

(If you draw any other inertial worldline in this spacetime diagram, that observer will assign unequal time-coordinates to A and Z.)

[Note: in Minkowski spacetime geometry, the segment PF is Minkowski-orthogonal to segment AZ.]

Consider 1+1 Minkowski spacetime.

Consider two events A and Z which are not causally related.

Their future light-cones intersect at an event (call it F) and
their past-light cones intersect at an event (call it P).

https://www.desmos.com/calculator/mveyzcilol

The inertial observer Bob through P and F observes that A and Z are simultaneous since Bob performs a "radar experiment" to assign a time-coordinate to event A by sending (when Bob's clock reads $t_P$) a signal to A and waiting for the arrival of its echo (when Bob's clock reads $t_F$). Bob assigns a time-coordinate to A using his clock readings $t_P$ and $t_F$: $$t_{A, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2}.$$ Similarly, $$t_{Z, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2},$$ which is equal to $t_{A, \rm accd\ to\ Bob}$.

(If you draw any other inertial worldline that is not parallel to PF in this spacetime diagram, that observer will assign unequal time-coordinates to A and Z.)

[Note: in Minkowski spacetime geometry, the segment PF is Minkowski-orthogonal to segment AZ.]

Consider 1+1 Minkowski spacetime.

Consider two events A and Z which are not causally related.

Their future light-cones intersect at an event (call it F) and
their past-light cones intersect at an event (call it P).

https://www.desmos.com/calculator/mveyzcilol

The inertial observer Bob through P and F observes that A and Z are simultaneous since Bob performs a "radar experiment" to assign a time-coordinate to event A by sending (when Bob's clock reads $t_P$) a signal to A and waiting for the arrival of its echo (when Bob's clock reads $t_F$). Bob assigns a time-coordinate to A using his clock readings $t_P$ and $t_F$: $$t_{A, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2}.$$ Similarly, $$t_{Z, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2},$$ which is equal to $t_{A, \rm accd\ to\ Bob}$.

(If you draw any other inertial worldline in this spacetime diagram, that observer will assign unequal time-coordinates to A and Z.)

Consider 1+1 Minkowski spacetime.

Consider two events A and Z which are not causally related.

Their future light-cones intersect at an event (call it F) and
their past-light cones intersect at an event (call it P).

https://www.desmos.com/calculator/mveyzcilol

The inertial observer Bob through P and F observes that A and Z are simultaneous since Bob performs a "radar experiment" to assign a time-coordinate to event A by sending (when Bob's clock reads $t_P$) a signal to A and waiting for the arrival of its echo (when Bob's clock reads $t_F$). Bob assigns a time-coordinate to A using his clock readings $t_P$ and $t_F$: $$t_{A, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2}.$$ Similarly, $$t_{Z, \rm accd\ to\ Bob}=\frac{t_F+t_P}{2},$$ which is equal to $t_{A, \rm accd\ to\ Bob}$.

(If you draw any other inertial worldline in this spacetime diagram, that observer will assign unequal time-coordinates to A and Z.)

[Note: in Minkowski spacetime geometry, the segment PF is Minkowski-orthogonal to segment AZ.]