Blizzard HAS said that they will be two different games.
Assume Retail and Classic are two separate games entirely (apart from the sub granting access to both) unless we announce otherwise.
Whether that means two different clients, I do not know. I would guess that they would be two different clients due to the different data, though. That is just my opinion.
And even though I am game for some changes from Vanilla at the BNet level, such as Classic -> Retail transmog unlocking (meaning that an item received in Classic will unlock for Transmog on Retail)
I believe that Blizzard has already indicated that this will NOT be happening.
I believe that they have said there would be no interaction between the two games.
I believe that Blizzard has indicted that what is obtained in one game remains in that game and does not affect the other game.
Classic the most authentic experience we can provide. Adding any sort of reward carry over in either direction isn’t authentic and defeats the point.
Classic is there for those that will want to play it. Not just for rewards on their BfA license, which would be devaluing to those that earned those rewards that are no longer obtainable, and vice versa would be introducing things that were never there in Classic to begin with. Neither of which we want to do in this case. We know the community that has been asking for Classic for years doesn’t want either of those scenarios, and neither do we.