[name_f]Annie[/name_f] [name_f]Fern[/name_f] is a pretty and unique first name, it’s no different to people being called [name_f]Mary[/name_f] [name_f]Kate[/name_f], etc, so I wouldn’t call it a far fetched idea. If you made it a singular first name, I’d suggest something like:
[name_f][/name_f][name_f]Annie[/name_f] [name_f]Fern[/name_f] [name_f]Elizabeth[/name_f]
[name_f][/name_f][name_f]Annie[/name_f] [name_f]Fern[/name_f] [name_f]Patricia[/name_f]
[name_f][/name_f][name_f]Annie[/name_f] [name_f]Fern[/name_f] [name_f]Amaya[/name_f]
[name_f][/name_f][name_f]Annie[/name_f] [name_f]Fern[/name_f] [name_f]Georgina[/name_f]
[name_f][/name_f][name_f]Annie[/name_f] [name_f]Fern[/name_f] [name_f]Felicia[/name_f]
I think using a 3 syllable name nicely balances with the 3 syllables in [name_f]Annie[/name_f] [name_f]Fern[/name_f] so it flows off the tongue very nicely.