When projectiles collide, only one gets destroyed

[Descen]

Noticed this on my map. I put two shooters at the opposite end of a corridor. Set them to shoot projectiles the same time. When the two projectiles meet at the middle of the corridor, only one vanishes(would say sort randomly chosen which one, at each map start) while the other keeps flying till it hit the the next solid entity or world brush.

This is my personal proposal (if possible), besides the normal collide with each other and both get destroyed:
One scenario where the projectiles ignore each other, and just pass thru till they hit something other solid. Maybe via some keyvalue (or spawnflag):
Collision: everything / everything, but projectiles