Green’s Dictionary of Slang

(gay) a sexual act in which the two partners alternately perform fellatio and sodomy on each other.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1965		Guild Dict. Homosexual Terms 16: fifty-fifty (adj.): To alternate between fellation and pedication; it is separate and distinct from sixty-nine (q.v.). Usually means, ‘I’ll fuck you first; then do [i.e. fellate] you.’.
1972		B. Rodgers Queens’ Vernacular 80: fifty-fifty 1. a split bill of ass-fucking and cocksucking; taking sexual turns with one another.