"killed them both" is correct and usable in written English.
It can be used when referring to the death of two people. For example, "The murderer killed them both in cold blood."
The second one killed them both".
"He killed them both for reasons which remain obscure.
"I've thought of her son every day since I found out that the same officer killed them both," she said.
Escobar and his bodyguard fired on the troops, who returned fire and killed them both, the ministry said.
"So," she said, "my father took my mother and brother down into the cellar and killed them both, before turning the gun on himself.
They heard one of the soldiers say, "Just kill them both".
What could Shakespeare do but kill them both at the end?
As his wife tries to save the third, he kills them both with a single arrow.
"If I became a college professor, I knew I was going to wind up killing them both off".
"The situation has become so irrational and so extreme," he joked darkly, "that we better kill them both before they kill us".
