answering own question
On reflection, to a (massless :-) observer at the center of mass of the two in-falling masses, the center of mass must remain stationary during the in-fall (because there is no force to move it), therefore the two masses must meet at the center of mass. This means the time for each mass to reach the collision point must be equal (which may make it easier to calculate the time it takes for them to arrive at the collision point?) Is this correct?