From a logistical point of view, having the same sprues sold under two different names/packages might be less than desirable. Not to mention potential for confusion/disappointment amongst persons buying both, expecting them to have separate contents. Perhaps other means of raising awareness amongst the fantasy crowd could be useful (say, big posters showing Celts fighting a dragon).
True, there are some logistical challenges. They may be worth it however, IF the resulting extra sales overall cover the additional costs. FREX, in my industry - which is still very much cash based - I see a lot our site owners balk at taking credit cards because of the credit card merchant fees - the merchant fees can run as high as 20% of sales, while credit cards only boost sales on average 15%. So they look at the 20% and 15% and think "That's a 5% loss, why would I want to do that?" What they typically don't calculate out is that 20% is actually on the extra 15%, typically resulting in a net 12% gain overall. 12% extra sales usually isn't something to sneeze at.
This could be a similar situation. Or not - I don't have all the facts to know for sure. But it's not unheard of for companies to market the exact same product under different names - particularly in consumer electronics. Many times DVD players, TVs stereos, etc, will be sold under different model numbers, and the only difference between them is the label with the model number, and the chain of stores they're sold at.
I was merely throwing the idea out there because it's "solution" to a "problem" I face. I often don't even bother with historical lines, even though I know there are some good candidate figures within them, simply because I don't know what I'm looking for and it gets frustrating. At one of my FLGSs, they have a nice historical display, and I can browse through them by sight, picking out what I want, so when I do by historicals, I usually go there. At another one, the entire historical section is nor more than a square foot worth of pegs, and consists primarly of WF zulu and British figures - because, in their words "historicals don't sell very well." And most online shops are organized to for the historical player who knows what he's looking for, not for the fantasy gamer who's looking for something appropriate for his fantasy army.
So my suggestion was really aimed at answering the question "How can we get more fantasy oriented stores to carry these and/or make them easier for Fantasy gamers to find?" It was by no means the only answer to that question.
Other possible 'solutions' could be posters/marketing campaigns like you suggest, add-on stickers for merchants/distributors to use to raise awareness as desire, dual sided packaging (one side says Historical Warriors, Type X, with one picture, while the other side of the box says "Fantasy Men-at-Arms"), etc.
My ideal solution would probably be to throw in a conversion sprue to make the vikings/celts/etc true Fantasy warriors, but I'm guessing that is probably the most expensive of the solutions, and not likely to happen for some time, if at all.