While I imagine the west will get a combo edition rather than the split edition(s), I'll still buy both versions if we get FE: WhiteGood and FE: BlackGoodButMisunderstood. My guess is the western release will be a combo pack regular release and a "collectors" release that comes with the third campaign as a DLC voucher or season pass or whatever.
They've more or less confirmed this to be the case. Japan will get Black Gold/Soul White editions in addition to a third combined release that will automatically lock out the side you didn't choose on your first playthrough (and requires DLC to unlock), and the fourth Collector's Edition that will be the combined release, but without the lockout, and a free pass to the inevitable Crystal Dragon Jesus (I'd blame TVTropes for this but after reading over half of GetBackers and seeing what the game is already implying, this is far more appropriate than it should be) version plus other feelies like an artbook.
The US at least will only be getting the third option of a combined game, but programmed to lock out half the content until DLC is bought (the handling of this is a large part as to why there are people getting pissed off with the release of this game). There's currently no word on a possible Collector's Edition release yet.
Card Fight VanEdit: And now we know what some of that DLC might be http://www.siliconera.com/2015/04/14/marth-lucina-fire-emblem-dlc-characters/
Also they're bringing the card game back.