I like your cute nicknames! For [name]Noa[/name]'s little sister – hmm, well, these are the ones that immediately spring to my mind, but I won’t be offended if you hate them all… other people here will probably have even better ideas!
[name]Noa[/name] and [name]Kai[/name] – [name]Kai[/name] has the unisex vibe that I think you like
[name]Noa[/name] and [name]Ani[/name] – not unisex but kind of cute and sassy
[name]Noa[/name] and [name]Sarai[/name] – I just love this name (pronounced Sa-[name]RYE[/name])
[name]Noa[/name] and [name]Clio[/name] – I like having a name with another O, but on an unstressed syllable so they sounds less heavy together (also: [name]Willow[/name], [name]Juno[/name])
Then there are a zillion names that end with A that sound really gorgeous, but maybe those are not quite your style?
I think there are lots of truly unisex names that could really work with [name]Noa[/name], but the last-as-first thing just seems a little too stylistically different for me. I’m thinking some of the nature-word names – [name]River[/name], [name]Sage[/name], [name]Juniper[/name] – might fit better, perhaps?