Desperate David has undertaken a dangerous desert trek rather than marry either of his current girlfriends. So each girlfriend (unaware of the others actions) sabotaged his reserve water supply.
Debra added a quick-acting poison while Dana punctured the barrel, allowing all of the poisoned water to run away.
One week later David died from dehydration.
When arrested Debra claimed she was innocent because even though she poisoned the water David did not drink any.
Dana claimed she was not guilty because puncturing the barrel had stopped David from dying sooner from the poison. Her action made him live longer.
They cannot be charged jointly since they acted independently
Who, if anyone, should be charged with what crime?
David died from dehydration, so Dana is guilty of murder in the 1st degree. Dana didn't know about the poison, so her defense is faulty... and if she had known about it, she would have still been guilty, because of her disregard for human life.
David might have died from poisoning (but didn't) so Debra is guilty of attempt to murder.