I personally like "Jaden" for the boy, and "Jenda" for the girl. The boy's name sounds a bit like "Gavin" (except it's pronounced "JAY-den"), and the girl's name has that pretty J-n-a sound you're also looking for. Plus, the two names go together, because they're anagrams of each other.
If you want to check out some other anagram possibilites for twin names, try this:http://behindthename.com/anagram.html
Paul Pan, the creator of the list, is none other than our own "PriaposLovs" who posts here. :)