My wife and I play in the same alliance, this morning we log in and see there’s little left of the 6* Titan, we team up to kill it. Before I’m able to get my 3rd attack in it is dead, yet neither one of us got the kill credit and I was the last one to attack. No change in kill number in my profile or hers.
To get a kill, you physically need to kill the titan during your attack. Meaning if you see it die, you get the kill. It’s possible for multiple people to get the kill (multiple attack and see it die) or zero (the cumulative score kills it but nobody kills it all together on their hit).
It sounds like neither of you actually killed the titan, but collectively, you hit it enough to kill it.