I love
Yvonne, but am not wild about
Yvette. "-Ette" names are not my cup of tea, as they're too girly.
Yvonne, on the other hand, sounds great on anyone of any age and just sounds timeless to me. If these names hard to prn, you could always change the spellings to
Ivonne and
Ivette (I'm assuming that it's the Y that's throwing you off).