Exactly. You just need to have WH1 in steam library. Things like Blood DLC will be separated for WH2 but will be free for owners of WH1 version. Old World campaign, Beastmen + Wood Elves minicampaiogns staying in WH1
News about combined map:
https://www.totalwar.com/blog/mortal-empires
Looks like we will get two different combined maps. Joined campaign for W1+2 is named "Mortal Empires". So list of campaigns:
The Old World - Included in Warhammer I
Eye of the Vortex - Included in Warhammer II
Mortal Empires - Free Campaign for Owners of Parts 1 & 2 (in WH2)
Warhammer III’s Main Campaign - Included in Warhammer III
A Very Big Campaign - Free Campaign for Owners of Parts 1, 2 & 3 (in WH3)
Because we have things like 40 level cap and multiplayer in WH2, WH1 factions for multiplayer and Mortal Empires will be updated and balanced for that. At least that would be my understanding.