Due to interference by a God at the moment of conception, these twins had different fathers — making one of them mortal and one immortal.
Their mother got eaten by cannibals before they were old enough to ask. So they had to watch each other’s backs, give the kiss of life and perform emergency operations after risky adventures just in case.
This seems to have worked so far and there are no obituaries. But even Godchecker can’t tell you which one of them is the God.