Hmm.
Eliana is one of those fast rising superstar names that blends right in with
Isabella,
Ella,
Anna,
Juliana,
Ariana,
Liliana,
Gabriella etc etc ad nauseam, whereas
Sunniva is unusual in its sound and etymology, and if it appears on a "popular names" list outside of Scandinavia any time soon I'll eat my hat. Both are very feminine, but where
Eliana is sugary sweet,
Sunniva has more of an edge to it. And where
Eliana fits right in with the Old Testament/Hebrew supertrend,
Sunniva is about a generation ahead with its Old English and Scandinavian connections. So in short...no, they don't fit.
Eliana's popular, and sounds more so, while
Sunniva is obscure. Now, if you're writing from Norway, that's a different matter, but in English-speaking countries I'd liken it to having brothers...oh,
Bertrand and
Brandon.
With
Eliana I'd think
Julia,
Isabella,
Tabitha. With
Sunniva I'd be thinking anything from
Ingrid and
Edith to
Ursula and
Ottilie.
You said you're iffy...so I hope you don't get upset about this. It's just my opinion!