While I don't think anything has been said officially, I would strongly suspect that save files from Warhammer 1 will not work with Warhammer 2. The campaign map is completely different as well as the factions available. Think of the campaign maps as completely seperate from each other. They do not add to existing ones and the combined map is a seperate campaign map that includes both landmasses from Warhammer 1 and 2.
Also: On save files I would recommend people to clear out their save file folder in "C:\Users\Username\AppData\Roaming\The Creative Assembly\Warhammer\save_games" every so often, as for me the save files average in about 10MB each.
Warhammer 2 is not an expansion adding new factions to the existing map in Warhammer 1. It has a seperate campaign map with 4 new factions, mechanics and goals. A few weeks after release a combined campaign will be added for free that combines the maps from Warhammer 1 and 2, Which factions will be playable in this combined campaign be be determined by whether you own Warhammer 1, 2, both and any faction DLC like "Realm of the Wood Elves".
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.