Frenk and Dyvud Retzun were from Janus, a world where a species starts as conjoined twins and eventually split into two identical beings. While this is an interesting biological process, it isn't exactly an impressive power. This fact, along with Frenk and Dyvud's inability to cooperate, led to the Legion's choice to reject Double-Header. The pair joined the Legion of Substitute Heroes, in an auxiliary status, but showed up for meetings infrequently. They did, however, manage to make an appearance at the Wedding Ceremony of Bouncing Boy and Duo Damsel.
Frenk and Dyvud apparently stayed on with the Substitutes, in the recently revealed continuity in Action Comics. It was mentioned that he was killed in Earth-Man's assault on the Subs, but this was never shown on-panel.