Cyndi Lu and Lyndi Su. The A team.
Lilye and Rosye.
Garry and Harry; or indeed Barry and Larry. (How about quads?)
Robert and Roberta. Stephen and Stephanie. Anton and Tonya.
Fred and Eric.
Ann and Grace.
Deborah and Melissa.
Harmony and Melody.
Maybe the Ann/Grace and Deborah/Melissa combos are too subtle?