Aside: Even if you only buy Warhammer 2, I suspect that when the combined campaign map gets released you will be able to play as Bretonnia as they were released as a free faction. Whether Bretonnia has a presence in the Warhammer 2 campaign we don't know yet. Though lorewise Bretonnia does have a presence in the Southlands with some crusaderstate-like settlements.
I think CA has said you only get the combined campaign map if you own both games (and, later, also if you own games 1 and 3, without needing #2).
I couldn't remember exactly. Here is an excert from the Warhammer 2 F.A.Q that clears the point.
Do I need the original game to play Total War: WARHAMMER II?No, you don’t need the first game in the trilogy in order to play WARHAMMER II. However, if you do own the first game as well as the WARHAMMER II, you’ll also be able to choose to play on a huge new combined campaign map (see below). So owners of both games will have three large and different campaign maps to choose from.
So it is as you say; You need to own both Warhammer 1 and 2 to play on the combined map.
@BuriBuriZaemon: The campaign in Warhammer: Total War is rather light on story with an eventual invasion by the forces of Chaos headed by Archaon. After that is dealt with the world continues on in whatever state it is left in with factions most likely going to war against each other now the big Chaos threat has, maybe, been defeated. (When Archaon shows up, all Order factions (Dwarfs, Empire etc) get a large bonus to diplomacy, banding together against Chaos. Afterwards the bonus is lost and most factions devolve into wars against each other).
In Warhammer 2 the campaign has a much more developed story. The vortex created by the High Elves millenia ago to syphon away excess magic has begun to destabilize. Each faction has a number of missions to achieve to complete rituals designed to either stabilize or destroy the vortex. This will most likely mean each faction will have to send armies all over the place to specific locations to complete these missions.
In Warhammer lore the vortex was created to help stem the original invasion of the Warhammer world by huge hordes of Daemons of Chaos. By draining away the magic the daemons are unable to materialize in the world except close to the north and south pole. It has not been mentioned whether any daemons will make an appearance in Warhammer 2.