DarthDavy: I imagine this will be the case.
I don't think GOG would want the buyers of the expansion pass feel fooled afterwards, but it does seem you're only winning 50 cents with buying the season pass.
Then again, should the pass holders save more in the first place? Instead of viewing them as two competing deals, what we really should do IMO is to see that they're two alternate ways of achieving the same goal - you save about 5€ if you pre-order both expansions. Same product, same delivery date, two different payment plans - pay for both right away, or pay for the first now and pay for the rest later.
The pass holders only get fooled if acquiring both expansions separately go noticeably below the 25€ mark - because that will mean those with "late" pre-orders actually get a better deal than "early" ones. But if that is not the case, there should be no issue. You get the expansion on day one, and you've paid no less or more than anybody else who also gets it on day one. It's only those who wish to wait 'til post-release who initially end up paying more.
Of course, amidst all this speculation... it's good to keep in mind that we don't actually know the regular price of Blood and Wine yet. While I'm fairly confident that I've figured it out, I'd still like a confirmation from the masters up high. And I really think they should give one, since I have it on a good authority that they've got themselves a sale if they do.