DekaMaster and DekaSwan (Dekaranger - 2004) / Shadow Ranger and Kat Ranger (PRSPD - 2005)
Even though we didn't see them fight together, Doggie Kruger and Swan Shitori were quite close. They are paired together in Gokaiger. Deka Master/Shadow Ranger are considered Sixth Rangers but Deka Break/Omega Ranger were considered Sixth Rangers more than anything. In Gokaiger, Deka Master is considered an Extra Hero, he has had many toys after him. Deka Swan only appeared in 2 episodes and Kat Ranger in one episode.
Magi Mother and Wolzard Fire (Magiranger - 2005) / White Mystic Ranger and Wolf Warrior (PRMF - 2006)
First female-male duo but it wasn't instant to know, even though they both appeared in the first episode. They were married. In the first episode, the white one fought the purple one when he was evil. In Magiranger, Wolzard seemingly killed MagiMother. In Mystic Force, Koragg just de-powered Udonna by taking her staff. They both had figures and their own re-colored MagiPhones in Magiranger. In Mystic Force, only Koragg got figures and very little compared to his Japanese counterpart. Both have Gasphon Ranger Keys for Gokaiger. Of the Japanese figures, you can have Magi Mother and Wolzard Fire as a duo.
Rio and Mele (Gekiranger - 2007) / Dia Shi and Camile (PRJF - 2008)
There has been many other female-male villain duos, but in Gekiranger, they were they first to be seen as protagnists and later be allies. Mele/Camile was the first villainess to have a 'zord' that combined with good zords to become giant robos. Mele never got her own action figure during the run of Gekiranger, but she did get a special figure for Girls in Uniform. Camile did get an action figure for Jungle Fury called 'Melle.' Mele did get a Ranger Key from Gashapon. Of the American toys, you can have Dai Shi and Camile together as a duo.
Go-On Wings (Go-Onger - 2008) / Ranger Silver and Gold (RPM - 2009)
The first opposite-sex main duos that were 'sixth rangers', Go-On Silver and Go-On Gold are brother and sister. Silver got a figure during Go-Onger's run and RPM. In Gokaiger, Gai/GokaiSilver unites their two Ranger Keys into one key and becomes Go-On Wings, without a skirt (from Silver). From other Japanese and American collections, you can have toys of both duos.