I think [name_f]Violet[/name_f] and [name_f]Juliet[/name_f] are fine together especially if you predominantly use nicknames. Some alternatives with a similar vibe that I think could be lovely with [name_f]Amelia[/name_f] and [name_f]Juliet[/name_f]:
[name_f]Stella[/name_f]
[name_f]Margot[/name_f]
[name_f]Sylvie[/name_f]
[name_f]Elodie[/name_f]
[name_f]Lenore[/name_f]
[name_f]Daphne[/name_f]