Me thinks you two need to trust your judgement!
Think about it this way ....... when you think of Tanzanite (top grade) you think of vivid, highly saturated blue rich Tanzanite. It's normally heated to get that colour. Some Tanzanites won't achieve that colour when heated so they're less valuable.
The blue/green variety is either (a) unheated or (b) heated but the colour and saturation hasn't achieved top grade status.
Is it rare? Not really - it's being sold on all the shopping channels and elsewhere so how rare can it be? If it were a "rare" stone would you honestly find one set in silver and being sold for not a lot of moolah?
A good bi-coloured Tanzy i.e. highly coloured, vivid, a large carat weight etc., will have a value but I've yet to seen one that's really vivid. You more often see large pieces but as Fluff said, they're wishy washy in colour.
The price of Tanzanite has dropped through the floor recently. The bi-colour ones are interesting but I would never buy them for rarity or because they may increase in value (highly likely they won't)!