Hmmm... you're right. I was originally thinking of it as two force vectors adding together to create a zero sum, but that wouldn't be right because they're two separate objects (if the cannonballs were perfectly adhesive to one another, then they'd create a single object and do what I originally thought).

In perfect conditions they'd each transfer their kinetic energy into the other and they'd fly apart at the same velocity. So yeah, very much like billiard balls. Good intuition.

I'm an electrical engineer, I only took static mechanics in school.