Haha I think it's more of an interspecies relationship, but as there don't seem to be any male viera, I don't know how they'd do it any other way...

I don't think there's so much romantic tension as a romantic connection between them the whole story. They seem to have an understanding which could come from a partnership. On the other hand, they never seem to argue, which could possibly be a point against that idea (depending on how cynical you are about relationships).