Symmetry of “Inertial Mass”

In any elastic collision between high velocity particles, conservation of momentum, expressed in its naive form in terms of the initial frame of either particle, requires that the “inertial” mass used for the other (and for both of the post-collision particles) include the relativistic gamma factor(s). So it is indeed true that twins moving at constant velocities in different directions BOTH infer the other’s inertial mass to be more than their own.

Source: (1000) Alan Cooper’s answer to Ok, Newton’s physics was wrong. But is there any experimental evidence suggesting Einstein’s relativity claim is true that twins moving at constant velocities in different directions BOTH measure the other’s inertial mass to be more than his own? – Quora