I wanted to indicate that 2 people really livened up the party so I said, "They were the lifes of the party." It seemed natural. Someone immediately suggested it should be, "lives of the party." This struck me as taking the word "life" too literally. Is there a (more) correct plural?
The "more correct" plural of 'life' is 'lives'. Given that a cat has nine lives, not nine lifes, one could say that two people are the lives of the party. On the other hand, as several have pointed out, you can stick with the singular life of the party. See the related When to use “lives” as a plural of life? and Why is the plural form of “life” “lives”, while the plural form of “still life” is “still lifes”?. Since you are dealing with an idiom, use lives as in a cat's nine lives. But ultimately it is opinion based.
A lively, amusing person who is the center of attention at a social gathering. For example, Eileen was the life of the party, telling one good story after another. [First half of 1800s]
Note: People sometimes replace party with other nouns. He gives the impression of having been the life and soul of the campus.
The term “life” refers to the party, not to the people who take part in it so the singular form is the appropriate term to use.