When two normal projectile hitboxes belonging to opposite players meet, both attack hitboxes erase each other.
In some cases, the neutralization process will only reduce the number of attack hitboxes; thus, it will reduce the potential hit count and that’s it.
There are other cases where one projectile completely outranks the other, and will just barrel on through it, wiping out one without being neutralized in any way itself.
Normally, strike hitboxes have damage hitboxes within them, so when opponents’ attacks hit each other, they will trade hits. If invincibility or clashing is involved, that’s a different matter.