To clarify, a collision has nothing to do with whos "arriving" first at all.
The kill credit is another story, that means who is getting the kill, who is dying "first". But that doesn't change the fact that when there's a collision on both screens, both take damage accordingly.
To succesfully "ram" an enemy while taking no collsion at all (=gaming the collision model), a player would have to aim for an empty point in air where he knows that on his enemies front end he and his enemy will collide.
To do that, you need to have the second sight. You don't know how things look on your enemy's computer. You don't now how big the combined lag is.
BTW, if "both go down" would be implemented, ramming would really work consistently for the first time, and it suddenly would be almost impossible to dodge them. It would be the reverse situation that we do have now.