Hmmn..
Twins eh? lots of stories about twins; tragedies, comedies, the works.
Boys; I'd go with the original Gemni, Castor and Pollux
For girls; Hera and Kara, if you want to be truely cruel you could refer to them as Hari-Kari
For a mix; gotta admit, Luke and Leia does work pretty well. There's always the option of just using the feminine/masculine of a name you like. Roy and Riza also seems to have a sort of
alchemy.
If I were you though, I'd talk it over with your significant other. Naming kids is really between the parents, it's none of our buisness.
Although we're honored that you'd ask.
Personally, I plan to name/call my kids Petra and Flint. Although my son's first name will actually be Frederick(counting in my last name it makes him Fred Flint Stone; although I imagine by the time I have kids people will only know of it as the vitamins).